“Even if we are doomed, and I’m totally naïve, I think it’s important that I do this work anyway,” said Mr. Teetsel, of the Manhattan Declaration. “If what I believe is true is true, then I’ve got a responsibility to be on its side for as long as I can be.”

 
They hear that their cause is lost, that demographics and the march of history have doomed their campaign to keep marriage only between a man and a woman. But the young conservatives who oppose same-sex marriage — unlike most of their generation — remain undaunted.

They identify themselves as part of the “pro-marriage movement” and see themselves at the beginning of a long political struggle, much like the battle over abortion. If they can begin shifting the terms of the debate away from gay rights and toward the meaning of marriage, they say, they have a chance to survive short-term defeats.

“The primary challenge that our side faces right now is the intense social pressure,” said Joseph Backholm, 34, the executive director of the Family Policy Institute of Washington. “To the extent that the other side is able to frame this as a vote for gay people to be happy, it will be challenging for us.”

To put it another way, opponents of same-sex marriage say they must argue in favor of traditional marriage, not against gay people or gay rights. “It’s really a broader defense of marriage and a stronger marriage culture,” said Will Haun, 26, a lawyer and member of the Federalist Society.

In the highest-profile effort, the National Organization for Marriage is gearing up for a march on the National Mall on Tuesday, the day the Supreme Court is scheduled to hear arguments on California’s 2008 voter-approved ban on same-sex marriage.

Last week, the Heritage Foundation released a report by Ryan T. Anderson, 31, in defense of traditional marriage, “Marriage: What It Is, Why It Matters, and the Consequences of Redefining It.” Mr. Anderson, a Heritage Foundation fellow, has also held briefings for members of Congress, their staff members and others to explain his arguments against same-sex marriage, and he and two co-authors released a book last year laying out their case in depth.

Still, the fight is shaping up to be a difficult one, with public opinion increasingly seeming to shift in favor of same-sex marriage. More than 100 high-profile Republicans recently filed a brief urging the Supreme Court to declare that gay couples have a constitutional right to marry. On Friday, Senator Rob Portman of Ohio became the only sitting Republican senator to publicly support same-sex marriage, citing his 21-year-old gay son, Will, as the catalyst.

Beyond Washington, polling has begun to show that a majority of Americans supports same-sex marriage, with even young Republicans moving in that direction.

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