At the center of the event was Robert P. George, a Princeton University professor of jurisprudence and a Roman Catholic who is this country's most influential conservative Christian thinker. Dressed in his usual uniform of three-piece suit, New College, Oxford cuff links and rimless glasses­, George convened the meeting with a note of thanks and a reminder of its purpose. Alarmed at the liberal takeover of Washington and an apparent leadership vacuum among the Christian right, the group had come together to warn the country's secular powers that the culture wars had not ended. As a starting point, George had drafted a 4,700-word manifesto that promised resistance to the point of civil disobedience against any legislation that might implicate their churches or charities in abortion, embryo-destructive research or same-sex marriage.

Two months later, at a Washington press conference to present the group's "Manhattan Declaration," George stepped aside to let Cardinal Rigali sum up just what made the statement, and much of George's work, distinctive. These principles did not belong to the Christian faith alone, the cardinal declared; they rested on a foundation of universal reason. "They are principles that can be known and honored by men and women of good will even apart from divine revelation," Rigali said. "They are principles of right reason and natural law."

Even marriage between a man and a woman, Rigali continued, was grounded not just in religion and tradition but in logic. "The true great goods of marriage - the unitive and the procreative goods - are inextricably bound together such that the complementarity of husband and wife is of the very essence of marital communion," the cardinal continued, ascending into philosophical abstractions surely lost on most in the room. "Sexual relations outside the marital bond are contrary not only to the will of God but to the good of man. Indeed, they are contrary to the will of God precisely because they are against the good of man."

George looked on with arms crossed and lips sealed. But he was obviously pleased. To anyone who knew George's work, the cardinal's words sounded very much as if George had written them, and when I asked him about it later, he acknowledged providing assistance. Rigali's remarks were a summation of the distinctive moral philosophy that is the foundation of George's power.

He has parlayed a 13th-century Catholic philosophy into real political influence. Glenn Beck, the Fox News talker and a big George fan, likes to introduce him as "one of the biggest brains in America," or, on one broadcast, "Superman of the Earth." Karl Rove told me he considers George a rising star on the right and a leading voice in persuading President George W. Bush to restrict embryonic stem-cell research. Supreme Court Justice Antonin Scalia told me he numbers George among the most-talked-about thinkers in conservative legal circles. And Newt Gingrich called him "an important and growing influence" on the conservative movement, especially on matters like abortion and marriage.

"If there really is a vast right-wing conspiracy," the conservative Catholic journal Crisis concluded a few years ago, "its leaders probably meet in George's kitchen."

FOR 20 YEARS, George has operated largely out of public view at the intersection of academia, religion and politics. In the past 12 months, however, he has stepped into a more prominent role. With the death of the Rev. Richard John Neuhaus, a Lutheran minister turned Roman Catholic priest who helped bring evangelicals and Catholics together into a political movement, George has assumed his mantle as the reigning brain of the Christian right. And he is in many ways the public face of the conservative side in the most urgent culture-war battle of the day. The National Organization for Marriage, the advocacy group fighting same-sex marriage in Albany and Trenton, Maine and California, has made him its chairman. Before the 2004 election, he helped a coalition of Christian conservative groups write their proposed amendment to the federal Constitution defining marriage as heterosexual. More than any other scholar, George has staked his reputation on the claim that same-sex marriage violates not only tradition but also human reason.

It's part of a philosophy that has found support among a group of Catholic bishops who have become some of the most persistent critics of President Obama and the Congressional Democrats. George serves as their intellectual point man. In the past few years, many of the evangelical Protestants who once defined the religious right have turned inward after their disappointment with President George W. Bush. In their place, George's friends among the Catholic bishops have stepped to the fore, hammering Obama for his pro-choice Catholic cabinet nominees, for being invited to speak at Notre Dame's commencement, for his stem-cell research policies and most recently for his health care proposals.

As Democrats have stepped up their explicitly religious appeals to Catholic voters, these bishops have pushed back against the intrusion on their turf. While Democrats talked of finding common ground on abortion, Archbishop Charles Chaput of Denver, the informal leader of this side of the American church, gave a much-publicized speech denouncing Obama as "the most committed abortion rights candidate in history." Chaput chose to publish his remarks on the Web site of a think tank co-founded by George - the man who had himself argued in an essay disseminated widely last fall through conservative circles, Fox News and Christian radio that Barack Obama was "the most extreme pro-abortion candidate ever to seek" the presidency.

George's role as an adviser to these bishops began more than 20 years ago, when he was a young professor and recent Ph.D. A mutual friend introduced him to the Rev. John Myers, then a bishop in Peoria, Ill., who was working on a pastoral letter about the moral obligations of Catholic voters and politicians. With George's assistance, Myers wrote a letter laying out the case that abortion, as the taking of a life, was a crime against the natural law of human reason, not merely a violation of Catholic theology. Therefore, Myers and George argued, Catholic politicians and voters were wrong to write off the church's teachings as a matter of personal faith. What's more, the letter warned, voting for a candidate or a law upholding abortion rights would almost invariably put a Catholic so far outside church teachings that he should not receive communion. As the first systematic rebuttal to Mario Cuomo and other Catholic politicians who support abortion rights, the letter kicked off a now-familiar debate inside the church. "Whenever I venture out into the public square, I would almost invariably check it out with Robby first," Myers, now the archbishop of Newark, told me. Many of the bishops, Myers says, rely on George as "a touchstone" and "the pre-eminent Catholic intellectual."

Last spring, George was invited to address an audience that included many bishops at a conference in Washington. He told them with typical bluntness that they should stop talking so much about the many policy issues they have taken up in the name of social justice. They should concentrate their authority on "the moral social" issues like abortion, embryonic stem-cell research and same-sex marriage, where, he argued, the natural law and Gospel principles were clear. To be sure, he said, he had no objections to bishops' "making utter nuisances of themselves" about poverty and injustice, like the Old Testament prophets, as long as they did not advocate specific remedies. They should stop lobbying for detailed economic policies like progressive tax rates, higher minimum wage and, presumably, the expansion of health care - "matters of public policy upon which Gospel principles by themselves do not resolve differences of opinion among reasonable and well-informed people of good will," as George put it.

A few months later, in a July 17 letter to Congress, the bishops did something close to that in the health care debate. Setting aside decades of calls for universal coverage, the bishops pledged to fight any bill that failed to block the use of federal subsidies for insurance covering abortion. "Stalin famously asked, 'How many divisions has the pope?' " George wrote to me in an e-mail message after House Speaker Nancy Pelosi allowed a vote on an amendment that satisfied the bishops' demands. "I guess Pelosi now knows."

In the American culture wars, George wants to redraw the lines. It is the liberals, he argues, who are slaves to a faith-based "secularist orthodoxy" of "feminism, multiculturalism, gay liberationism and lifestyle liberalism." Conservatives, in contrast, speak from the high ground of nonsectarian public reason. George is the leading voice for a group of Catholic scholars known as the new natural lawyers. He argues for the enforcement of a moral code as strictly traditional as that of a religious fundamentalist. What makes his natural law "new" is that it disavows dependence on divine revelation or biblical Scripture - or even history and anthropology. Instead, George rests his ethics on a foundation of "practical reason": "invoking no authority beyond the authority of reason itself," as he put it in one essay.

George's admirers say he is revitalizing a strain of Catholic natural-law thinking that goes back to St. Thomas Aquinas. His scholarship has earned him accolades from religious and secular institutions alike. In one notable week two years ago, he received invitations to deliver prestigious lectures at the Southern Baptist Theological Seminary and Harvard Law School. His critics, including many of his fellow Catholic scholars, argue that he is turning the church into a tool of Republican Party. They say he is too focused on the mechanics of sex and morality, neglecting the other sides of the Christian message: the corruption of human reason through original sin, the need for forgiveness and charity and the chance for redemption. Citing George's comparison of Catholic scholars who support abortion rights to defenders of chattel slavery, Cathleen Kaveny of the Notre Dame Law School, another scholar of law and theology in the Thomistic tradition, has called George and his allies "Rambo Catholics" and "ecclesiastical bullies."

I met George 20 years ago, when I was a Prince­ton student and he was praying for tenure. The next time I saw him was in 2005, when he invited me back for a postelection conference on the future of the conservative movement. Rove spoke at lunch. To a movement still stinging from decades of condescension, George brings gleaming Ivory Tower credentials: degrees in law and theology from Harvard; a doctorate in philosophy of law from Oxford; a Supreme Court fellowship; and the endowed chair at Princeton that Woodrow Wilson once held.

In late August I returned to Princeton, where George was striding around a seminar room at the Princeton Theological Seminary - "John Paul II style," as he put it, a reference to the pontiff's habit of strolling as he spoke. At 54, George has thick gray-brown hair, bright blue eyes and a certain boyishness. Seemingly everyone from Rove to Cardinal Rigali calls him, simply, Robby. A few dozen graduate and star-undergraduate students had traveled from as far away as Cambridge and Poland for a seminar on the new natural law. He is by all accounts a terrific teacher. ("Awesome," several undergraduates said in a stack of glowing evaluations he showed me.) Part of the reason may be that he brings almost every philosophical question back to a central debate about the nature of the self, a battle between reason and the passions. Moral philosophy, as George describes it, is a contest between the Greek philosopher Aristotle and the Scottish enlightenment thinker David Hume.

Aristotelians, like St. Thomas Aquinas, hold that there is an objective moral order. Human reason can see it. And we have the free will to follow or not. "In a well-ordered soul, reason's got the whip hand over emotion," George told the seminar, in a favorite formulation borrowed from Plato. Humeans - and in George's view, modern liberals are usually Humeans - disagree. Against Aristotle, Hume argued that the universe includes facts but not values. You cannot derive moral conclusions from studying the world, an "ought" from an "is." There is no built-in, objective reason for me to choose one goal over another - the goals of Mother Teresa over the goals of Adolf Hitler, in George's hypothetical. Reason, then, is merely a tool of whatever desire strikes my fancy. "Reason is and ought only to be the slave of the passions and may pretend to no office other than to serve and obey them," George said, paraphrasing Hume, just as he does in seemingly every essay or lecture he writes.

In George's view, if I have no rational basis for picking one goal over another, then I have no free choice, only predetermined "passions" - the result of genetics, a blow to the head, whatever made me prefer either curing the sick or killing the Jews. We have reason and free choice, he teaches, or we have amorality and determinism.

George's thinking draws on a system of ethics first developed against the backdrop of the 1960s debate inside the Catholic Church over contraception. In the tradition of Aquinas, Catholic thinkers had for centuries tried to establish moral laws of nature by studying biology, anthropology and history. When it came to sex, the church taught the idea of a "perverted faculty." Sex was intended for the dual purpose of procreation and marital unity, so deliberate ejaculation in any other context - oral or anal sex, artificial contraception, masturbation, premarital sex, adultery - demeaned sex and contravened the natural law. (Female orgasms, incidental to conception, were not an issue.)

But by the late 1960s, most secular philosophers had abandoned the project of finding moral norms in nature. Amid the openness of the Second Vatican Council, some of their Catholic counterparts began to wonder if they should give up, too. Then came the pill. Some Catholic scholars, all the way up to the level of papal advisers, questioned whether a form of birth control that did not put a physical barrier between the partners might be permissible. Some began to suggest that the church should shift its focus from the act of sex to the totality of marriage, as Protestants did, and stop worrying as much about ejaculation and contraception. Wasn't it marital love that was meant to be fruitful and that gave sex its meaning?

An orthodox-minded Georgetown University philosopher named Germain Grisez mounted a novel defense of the birth-control ban. Instead of beginning with science or history, he started by listing certain basic human goods that he believed anyone could see were "integral to human flourishing," like friendship, knowledge, excellence in work and play, religion, life and procreation. Each was an end in itself, not a means to anything else. You could never prove each's value by referring to other values - only assert and defend each one on its own.

Grisez argued that contraception violated the "basic good" of "the handing on of new life." For George and the new natural lawyers, Grisez's tactic of starting from self-evident human goods gave "the whip hand" back to reason.

In practice, George and his allies have usually found the rules of sexuality quite absolute, while the church's teachings about social justice come out more contingent. That may be why he is almost uniformly popular among evangelicals but controversial among many of his fellow Catholics, particularly those who prefer the church's peace-and-justice liberalism to its conservative bioethics.

On the question of capital punishment, George says he is against it but he considers it a matter of interpretation about which Catholics can disagree. The intentional killing of innocent civilians in war is as grave a moral crime as abortion, George says, but what constitutes a "just war" is a more complicated judgment call. Before the 2003 invasion of Iraq, he wrote an op-ed article in The Wall Street Journal arguing that the attack was not necessarily unjust and might even be a moral obligation. "On the evidence that Hillary Clinton voted for the war on and George Bush went to war on, I thought it was justified," he told me.

The "rights" to education and health care are another matter, George told his seminar. "Who is supposed to provide education or health care to whom?" George asked. "Health care and education are things that you have to pay for. Resources are always finite," he went on. "Is it better for education and health care to be provided by governments under socialized systems or by private providers in markets or by some combination?" Those questions, George said, "go beyond the application of moral principles. You can get all the moral principles dead right and not have an answer to any of those questions."

But the argument for banning abortion and embryo-destructive research is "straightforward," George told me several times. In his most recent book, "Embryo," written with Christopher Tollefsen, George tells the story of Noah Benton Markham, rescued from Hurricane Katrina by a team of policemen in boats. Noah was an embryo frozen in liquid nitrogen on a hospital shelf. Later implanted in the womb of his biological mother, he will turn 3 next month. Science shows that you remain the same human with the same DNA as a teenager, a toddler and an embryo, George argues. The only moral debate, he says, is whether you deserve legal protection at each stage of your life.

George grew up in Morgantown, W.Va., the oldest of five brothers. All served as altar boys. Four attended Oxford and became lawyers. His father was a liquor broker, and George is a wine connoisseur. His mother, the daughter of Italian immigrants, taught her children "some pretty firm ideas about sexual morality," George told me, and then he begged me not to repeat some of his more recent arguments on the subject. "Mom, I have got to explain." George said, raising his voice to imitate first himself and then his mother: " 'George's opposition to sodomy. What are you doing talking about sodomy? You shouldn't even know what that is. Why do people have to know your views about that?'

"The Roman Stoic Musonius Rufus said, Look, there are some things you just shouldn't have to talk about, and I think sex is one," George said. "But in the circumstances we are in, what can you do? I can't possibly make the case - to people on my own side or on the other side - about what I think marriage is and why it is so important unless we get into the earthy stuff."

The same-sex marriage debate, George argues, illuminates an error in our understanding that he blames for most of the ills afflicting modern marriage - infidelity, divorce, out-of-wedlock births. Marriage is not just for procreation, love or sexual pleasure. "People have lost their grip on the true reasons for marrying, so they are unwilling to make all the sacrifices real marriage requires," he said.

He admits the argument for marriage between a man and a woman can require "somewhat technical philosophical analysis." It is a two-step case that starts with marriage and works its way back to sex. First, he contends that marriage is a uniquely "comprehensive" union, meaning that it is shared at several different levels at once - emotional, spiritual and bodily. "And the really interesting evidence that it is comprehensive is that it is anchored in bodily sharing," he says.

"Ordinary friendships wouldn't be friendships anymore if they involved bodily sharing," he explained to me. "If I, despite being a married man, had this female friend of mine and I said, 'Well, gosh, why don't we do some bodily sharing,' and we had straightforward sexual intercourse, well, that wouldn't be friendship or marriage. It is bodily, O.K., but it is not part of a comprehensive sharing of life. My comprehensive sharing of life is with my wife, which I just now violated." But just as friendships with sex are not friendships, marriage without sex is not marriage. Sex, George said, is the key to this "comprehensive unity." He then imagined himself as a man with no interest in sex who proposed to seal a romance by committing to play tennis only with his beloved. Breaking that promise, he said, would not be adultery.

The second step is more complicated, and more graphic. George argues that only vaginal intercourse - "procreative-type" sex acts, as George puts it - can consummate this "multilevel" mind-body union. Only in reproduction, unlike digestion, circulation, respiration or any other bodily function, do two individuals perform a single function and thus become, in effect, "one organism." Each opposite-sex partner is incomplete for the task; yet together they create a "one-flesh union," in the language of Scripture. "Their bodies become one (they are biologically united, and do not merely rub together) in coitus (and only in coitus), similarly to the way in which one's heart, lungs and other organs form a unity by coordinating for the biological good of the whole," George writes in a draft of his latest essay on the subject. Unloving sex between married partners does not perform the same multilevel function, he argues, nor does oral or anal sex - even between loving spouses.

Infertile couples, too, are performing this uniquely shared reproductive function, George says, even if they know their sperm and ovum cannot complete it. Marriage is designed in part for procreation in the way a baseball team is designed for winning games, he says, but "people who can practice baseball can be teammates without victories on the field."

George argues that reason alone shows that heterosexual sodomy and homosexual sex are morally wrong, just as the Catholic Church, classical philosophers and other religious traditions have historically taught. Unlike marital union in his special sense, he contends, such acts treat the body as an instrument of the mind's pleasure. As both a practical and a philosophical matter, he argues, the law should not necessarily police such things. But the need for the state to establish a proper definition of marriage is a different matter, he says, because the law has always regulated it in the interest of parenthood and community. "Marriage in principle is a public institution," he said. "I don't think it can be like bar mitzvahs or baptisms or the Elks Club."

It is safe to say that not many contemporary philosophers - whether secular or Catholic - agree with George's marriage argument. Many balk at the mystical "unitive and procreative" qualities George ascribes to sexual intercourse. The idea of "one flesh" union seems far less obviously intelligible than other "basic goods" like friendship, knowledge or religion. Even fellow Catholic Thomists who oppose same-sex marriage question the esoteric quality of George's argument. Why not just begin with the fact that humans are sexual and ask how best to channel that sexuality? Liberals, on the other hand, generally argue that the meaning of marriage is in the partners' love, not their loins. (To which George counters that they offer no definition that would exclude polygamy.)

GEORGE ONCE WON two terms as governor of the West Virginia Democratic Youth Conference in high school and even served as an alternate delegate to the 1976 Democratic convention. He moved right in the 1980s, initially over the issue of abortion, which eventually took him back to politics. On the day of the Pennsylvania primary in the 1992 presidential campaign, Bob Casey, then the state's governor (and the father of the current Senator Casey) surprised George with a phone call to talk about George's criticisms of Mario Cuomo. Later that year, when Bill Clinton denied Casey a chance to speak about abortion at the 1992 Democratic convention, it was George who had helped to write Casey's speech. Two years later, George represented Mother Teresa before the Supreme Court in an amicus brief seeking the overturning of Roe v. Wade. When George W. Bush became president in 2001, George was an active player in weekly White House conference calls for Catholic allies. Bush later awarded George a Presidential Citizens Medal. During the 2008 campaign, Rudy Giuliani, Mitt Romney and John McCain each sought George's counsel.

At Princeton, George founded an independently financed center, the James Madison Program in American Ideals and Institutions. Conservatives celebrate it for bringing conservative thinking to the campus and for adding "Princeton" to the résumés of young scholars. In the decade since its founding, George has helped scholars and foundations use it as a template for similar outfits at Georgetown, New York University and Williams College. Last spring, he joined a group of undergraduates in their call for a new university Center for Chastity and Abstinence. (He suggested they might have better luck with the name "Center for Love and Fidelity.")

George's left-leaning colleagues say he is unfailingly polite and even helpful. He co-teaches­ a great-books seminar with the African-American scholar Cornel West, who told me he thinks of George as "just a nice brother." George and his wife, Cindy, who is Jewish, socialize with liberal professors. But his relationships with other Catholic scholars are sometimes more contentious, especially about politics.

Shortly after the last presidential election, John Haldane, an eminent Thomist who is a friend of George's and an adviser to the Vatican, sent an open "Letter to America" to a Web site of the Witherspoon Institute, a conservative think tank that George helped found. Citing issues like the Iraq war and the Wall Street debacle, Haldane argued that Obama won in part because American voters "took moral exception to some of the policies pursued by the Bush administration, and I believe that in this their judgment was correct."

Haldane, an orthodox Catholic opponent of abortion and same-sex marriage, argued that his fellow "moral conservatives" like George risked their own credibility if they continued attacking Obama as morally inferior. When I reached him by phone in Scotland, where he lives, Haldane told me that he could not have recommended voting for either candidate in America's 2008 election. "If you were going to vote for John McCain, how are you going to square that with these discussions about social justice - about the running sore of structural deprivation running through American society, or the prosecution of an unjust war?" he asked. "There has been a tendency of social conservatives to either hold their noses because they are more concerned about abortion, or just not to notice the smell."

When I asked George about the letter, he was derisive. "John, thanks for the advice." he said sarcastically. "Gosh, I wish we would have taken it. We would have the strong and vibrant social conservative movement that you guys have in Great Britain."

George instead is plunging deeper into partisan politics. Alarmed at signs that the Republican Party was moving away from cultural issues, he recently founded a new group called the American Principles Project, which aims to build a grass-roots movement around his ideas. "His new venture will make him a major political player," the conservative writer Fred Barnes predicted in The Weekly Standard. Among the group's first endeavors has been to call for the ouster of Kevin Jennings, an Obama education official who previously founded the Gay, Lesbian and Straight Education Network. Jennings, George says, wants to "use our elementary schools in defiance of the wishes of parents, to use our elementary schools to teach pro-sexual-liberationist, pro-homosexualist propaganda."

I asked George several times if he was really hoping to ground a mass movement in abstract principles of reason so at odds with the prevailing culture. It was a bet, he said, on his conviction about the innate human gift for reason. Still, he said, if there was one critique of his work that worried him, it was the charge that he puts too much faith in the power of reason, overlooking what Christians describe as original sin and what secular pessimists call history.

It is a debate at least as old as the Reformation, when Martin Luther broke with the Catholic Church and insisted that reason was so corrupted that faith in the divine was humanity's only hope of salvation. (Until relatively recently, contemporary evangelicals routinely leveled the same charge at modern Catholics.) "This is a serious issue, and if I am wrong, this is where I am wrong," George acknowledges.

Over lunch last month at the Princeton faculty club, George noted that many evangelicals had signed the Manhattan Declaration despite the traditional Protestant skepticism about the corruption of human reason. "I sold my view about reason." he declared. He was especially pleased that, by signing onto the text, so many Catholic bishops had endorsed his new natural-law argument about marriage. "It really is the top leadership of the American church," he said.

"Obviously, I am gratified that view appears to have attracted a very strong following among the bishops," he went on. "I just hope I am right. If they are going to buy my arguments, I don't want to mislead the whole church."

----David D. Kirkpatrick is a correspondent in the Washington bureau of The New York Times.