WASHINGTON — 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.