Conservative evangelical Christians have a warning for any Republican who wants to be president: you’d better not support gay marriage.

Religious GOP activists say that gay marriage nationwide is not inevitable, despite a decision by the Supreme Court on Monday not to take up a series of federal court rulings on on the issue, effectively legalizing it in 11 states. And evangelical leaders, a powerful force in early presidential primaries in Iowa and South Carolina, say they will oppose any candidate who will not say unequivocally that marriage is only between a man and a woman.

“We believe that there are non-negotiable values, marriage being one,” said Bob Vander Plaats, a social conservative activist in Iowa who endorsed and organized support for the last two winners in the caucuses there, Mike Huckabee and Rick Santorum.

He added, “It’s going to be more than a talking point, a lot of time in the past people have gotten by without saying much. Now, it’s going to be ‘what is your leadership strategy to ensure that marriage is upheld, one man, one woman for the benefit of the entire country?'”

Russell Moore, the head of the public policy arm of the Southern Baptist Convention, which includes congregations that have a combined 16 million members, said, "I don’t think you will see social conservatives supporting a candidate who will reject the traditional definition of marriage.”

Huckabee himself, in an interview Thursday with the conservative American Family Association, said he would consider leaving the GOP and becoming an independent if it won’t fight gay marriage

Those comments reflect the complicated dynamic Republican candidates will face when the presidential campaign starts in earnest after November. Support for gay marriage among Americans overall is on the rise. According to data from the Pew Research Center, 60 percent of Americans opposed same-sex unions in 2004, but only 41 percent do now. But among self-identified white evangelicals, 85 percent opposed gay marriage in 2004, and 75 percent still hold that view.

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GOP candidates are caught between a majority of all voters, particularly younger ones, who view opposition to gay marriage as a form of bigotry and an older, traditional part of their base that believes gay marriage violates their Christian faith.

For now, nearly all of the potential 2016 Republican candidates say they do not support gay marriage. But when pressed on the issue, they often want to change the subject quickly, noting that court rulings or polls clash with their personal views.

In response to the Supreme Court’s moves, which cleared the way for gay marriages in Wisconsin, Republican governor and potential White House candidate Scott Walker said the debate in his state is “over,” even as he noted he still supports a ban on same-sex unions.

We believe that there are non-negotiable values, marriage being one

Rand Paul, in a recent interview with CNN, when asked if he could “rethink” his current opposition to gay marriage, raised his hands in the air and wouldn’t answer, looking frustrated by the question but not rejecting its premise.

Paul, a libertarian on some issues, seems the most likely Republican candidate to break with the party’s orthodoxy and back same-sex marriage. The question is whether religious voters in the Republican Party, particularly in early primary states, would support a conservative on other issues who also backed gay marriage – or whether such a stance would doom any GOP hopeful’s candidacy.

“Could Rand Paul win if he was not opposed to gay marriage? I don’t know the answer to that,” said Craig Robinson, former executive director of the Iowa Republican Party. “In terms of the whole libertarian crowd, there’s a large coalition of people who just don’t want that issue to be a government issue.”

He added, “I don’t think it’s a deal breaker.”

Robinson argued there was a path to winning the Republican nomination without being conservative on social issues by appealing to the kind of voters who backed Mitt Romney in 2012. Those more moderate Republicans might either support gay marriage themselves or simply not regard the issue as a litmus test. New Jersey Gov. Chris Christie has said he opposes gay marriage, but he may be in a position politically to rethink that position, since his path to victory in a GOP primary will likely be in states like New Hampshire and Florida that are not dominated by evangelical voters.

But Vander Plaats and other conservatives say they are determined not to give GOP candidates space to downplay this issue. Socially conservative leaders say they plan to press, both publicly and in private meetings with the candidates, that Republicans commit to opposing gay marriage early in the race.

And these activists have sway, as the candidates themselves have already illustrated. Moore has already met in small sessions with some of the potential candidates, including ex-Florida governor Jeb Bush. In August, Texas Sen. Ted Cruz, Louisiana Gov. Bobby Jindal, Texas Gov. Rick Perry, Huckabee and Santorum all attended an event put on by Vander Plaats’ group, The Family Leader.

“There is no way under any circumstances I would vote for a candidate that won't protect my most basic God-given rights enshrined in the Constitution, which the redefinition of marriage threatens,” said Steve Deace, a conservative radio talk host in Iowa. Deace added that opponents of same sex marriage have already experienced “assaults on the free speech, private property rights, and religious liberty of those who believe in marriage as we've always known it.”

Marriage laws should reflect the values and mores of the citizens of each state. Not unelected judges: https://t.co/5xAntmFUXX — Senator Ted Cruz (@SenTedCruz) October 8, 2014

The emergence of Cruz could make it easier for GOP activists to force other candidates to stick with the party’s current opposition to gay marriage. The son of a pastor, Cruz condemned Monday’s decision by the Court immediately after it was announced as “judicial activism at its worst” and called for a constitutional amendment that would allow states to keep in place gay marriage bans.

If Cruz elects to run for president with this firm stance on marriage, Paul and others may be reluctant to take a more liberal position and risk the ire of conservatives.

“I’m loathe to say never, but I can’t imagine that being the case,” said Chuck Hurley, another social conservative in Iowa, when asked about supporting a candidate who is for gay marriage.

Noting the strong opposition to gay marriage from Cruz, Perry, Jindal and Huckabee, he said, “we have excellent likely candidates who understand that kids should have a mom and a dad.”