THRILLING as it was to hear a sitting president endorse marriage rights for gay and lesbian Americans, there is one man whose opinion on the subject matters more than President Obama’s. Justice Anthony Kennedy is the likely deciding vote when this issue reaches the Supreme Court. More than anyone, he has the power to transform what is now a license bestowed by the more enlightened states into an all-American civil right.

Kennedy’s role as the court weather vane has given rise to a species of argument that Jeffrey Rosen, the George Washington University law professor, calls “the Kennedy brief,” in which “lawyers on both sides fall over themselves to court Kennedy’s favor by repeatedly citing the opinions of Justice Kennedy.” I’ve been trying to envision the Kennedy brief in defense of gay marriage.

There is plenty of Kennedy to cite on the subject. Although he is a devout Catholic, he is the author of the two most important pro-gay-rights decisions handed down by the court. In 1996, in a case called Romer v. Evans, Kennedy wrote an opinion ending Colorado’s attempt to repeal local laws protecting gays against discrimination. There was no plausible explanation for Colorado’s action, he wrote, beyond “animus” — judicial language for plain bigotry. In his 2003 majority opinion in Lawrence v. Texas, Kennedy and company threw out all remaining state laws against sodomy, saying that how consenting homosexuals express their love is none of the state’s business. (The verdict prompted Justice Antonin Scalia, in an indignant dissent, to warn that Kennedy’s argument “dismantles” any constitutional case against gay marriage.)

Now two lawsuits are inching toward the Supreme Court. The richer opportunity comes from California, where Proposition 8, banning gay marriage, has already been overturned by a three-judge panel of the United States Court of Appeals for the Ninth Circuit.

The Prop 8 ruling was a great victory for proponents of marriage equality, but it was a narrow victory, confined to California’s circumstances. (The state legalized gay marriage for a time, then took away that right.) That leaves the justices with some options:

They can make history. They could rule that the Constitution affords gay couples the same marriage rights as straight couples. Or they could take a big step in that direction by ruling that while states do not have to issue licenses to gay couples, they must recognize marriages performed in more progressive states.

They can thwart history. They could rule that states are free to recognize only heterosexual marriages. This would be a disheartening setback, though I imagine it would energize the movement for marriage rights.

They can postpone history. This is the easy way out. The court could endorse the narrowly worded appellate court ruling on Prop 8, either by actively affirming it or simply declining to hear it. By restoring marriage rights in California, this would instantly double, to 23 percent, the percentage of Americans living in states that treat gays as equals. But it would put off the day when this right is afforded to all Americans.

Ever since Proposition 8 entered the courts, advocates of gay marriage have been tactically divided about how much to ask for, how fast.

The most prominent champions of a sweeping claim to equality are David Boies and Ted Olson, the improbable Democrat-Republican duet, who were famously the rival lawyers in Bush v. Gore. Interviewing them in a public forum recently, I pressed for their reading of the current court, and specifically Justice Kennedy. They prudently ducked the question. But they have been buoyantly, insistently optimistic about the chance of winning a majority of the Supreme Court. They often cite Kennedy’s rulings in Romer (to argue that exclusive marriage laws are another example of mere bigotry) and in Lawrence (where Kennedy contends that a long history of moralistic disapproval is not, by itself, sufficient basis for outlawing behavior).

In contrast to constitutional literalists like Justice Scalia, Kennedy believes the court should track evolving standards in society. The Kennedy brief can argue that the incremental but steady progression of marriage equality laws — and the polls showing accelerating public approval — are evidence that the nation is evolving toward an obvious outcome.

Moreover, Justice Kennedy, who is widely traveled and cosmopolitan, is more open than the most conservative justices to arguments borrowed from foreign and international courts. He took note of European precedents in the ruling against sodomy laws. So the Kennedy brief on marriage could draw on the experiences of the 10 countries, from Canada to Spain to South Africa, that have legalized gay marriage, and the others (Israel and Mexico for instance) that recognize gay marriages performed in other jurisdictions. It’s hardly a worldwide consensus, but the trend is in the right direction, and, really, why should the United States lag behind Argentina?

Of course, one of the most potent arguments for pushing the court to be bold on this issue will not be heard in any courtroom: next January gays could face the prospect of President Romney. Mitt Romney not only opposes gay marriage and civil unions; he is committed to filling any Supreme Court vacancies with Scalias.

Perhaps the most influential advocate of a slower course is William Eskridge Jr., a Yale law professor, copious writer and litigator in support of gay rights, and author of an amicus brief in the Prop 8 case that reads very much like what the appeals court actually decided.

Eskridge, too, has studied Kennedy, and notes that the justice’s passion for liberty is matched by his fear of getting too far in front of the social consensus. By the time of the Lawrence case, states had either repealed anti-sodomy laws or were not enforcing them. In contrast, most states have constitutional amendments or statutes limiting marriage to heterosexual couples, and many are of recent vintage.

“Kennedy does not vote in a way that he thinks is going to undermine the legitimacy of the court,” Eskridge told me. Thus Kennedy has gone out of his way to stress that his earlier decisions did not necessarily lead to marriage equality.

Eskridge supports gay marriage, agrees that it is inevitable, but believes it would be counterproductive, even dangerous, to force it on a nation that is still evenly and intensely divided. He contends that — like the Roe v. Wade legalization of abortion — a Supreme Court ruling in favor of nationwide gay marriage would inflame hostility, even violence, especially in the Southern states. In the South the prejudice endures, foolish but devout, that families headed by gay couples are harmful environments for children.

“I’m a fan of Ted Olson,” he told me. “With all due respect, I think he’s wrong in 2012. In 2021 he’s right. But it’s going to be after you get states like Illinois and Colorado and Arizona and Wisconsin recognizing same-sex marriage.”

That, by the way, seems to be President Obama’s view, too: supportive of gay marriage, but not, so far, an advocate of forcing it on the states that are firmly opposed.

“We will still have same-sex marriage,” Eskridge said. “But not in Mississippi for a while.”

Professor Eskridge is a gay man who divides his time between New Haven and Washington, two cities where same-sex marriage is legal. I asked whether he felt any pangs about saying to a gay couple in West Virginia, where he grew up: “Just wait another decade or so, and you can have the same rights I have.”

“America is a mobile country,” he replied. “You notice that I am not living in West Virginia anymore.”

My head tells me that Eskridge is probably right: Kennedy will be inclined to stop short of full equality now. But my heart will be rooting for Boies and Olson. If that lesbian or gay couple in Mississippi or West Virginia have the courage to ask for a marriage license, I’d like to think the country has the courage to back them up.