So Ohio Senator Rob Portman is considering a run for president, and he claims his support for gay marriage would be a plus in a general election, allowing Republicans to make an economic case to key demographics that are culturally resistant to the GOP. “You can’t become a national party unless you do a better job reaching those between 18 and 30,” Portman says.

This raises the possibility of a scenario that Republicans who agree with Portman — and believe the party must evolve on gay marriage to stay in step with the country’s cultural and demographic shifts — might want to start worrying about right about now.

It’s not hard to imagine that Senator Ted Cruz might offer precisely the opposite case from Portman, making the argument that the party must reaffirm its support for “traditional marriage” key to his GOP presidential primary run. This could come after the Supreme Court has declared a Constitutional right to gay marriage — which Cruz would then be vociferously calling on Republicans to help roll back.

Gay advocates believe lower court rulings overturning state gay marriage bans on Constitutional equal protection grounds could portend an eventual SCOTUS ruling that enshrines a national right to gay marriage. That could happen in time for the 2016 primary.

That would amount to a powerful declaration that this debate is, or should be, culturally and legally settled. But at that point, unrepentant foes of gay marriage could seize on the ruling to redouble their call for a Federal Marriage Amendment to the Constitution defining marriage as between a man and a woman. Such a measure was introduced by House Republicans as recently as last year. And Senator Cruz supports the idea.

If you don’t think Cruz would love to demagogue such a SCOTUS ruling — and will demagogue the issue whether or not there is any such SCOTUS ruling — then you haven’t been paying close attention to the good Senator from Texas. In a recent speech reported by Jeffrey Toobin, he strongly suggested he would make this cause central to any presidential run, and threw in criticism of SCOTUS on it:

“Marriage is under assault…We’re seeing marriage under assault in the courts, including, sadly, the Supreme Court of the United States…I’m going to encourage each and every man and woman here to pray. If ever there was an issue on which we should come to our knees to God about, it is preserving marriage of one man and one woman…we need to turn this country around…we’ve got an election coming up in 2014, and, let me tell you, it’s going to be phenomenal. We’re going to retake the U.S. Senate! And I’ll tell you this: as good as 2014’s going to be, 2016’s going to be even better!”

Many Republicans have in effect stood on the sidelines throughout the country’s cultural transformation on gay marriage, but coming after a SCOTUS ruling, such a stand would come across as an active effort to move the country backwards.

Whether or not there is any SCOTUS ruling, there are already signs gay marriage could divide Republicans in 2016. The RNC autopsy into what went wrong in 2012 explicitly called for evolution on the issue, in part to keep in step with the cultural sensitivities of young Republicans and conservatives. Meanwhile, GOP-aligned gay advocates are actively planning to encourage pro-marriage equality voices within the GOP to speak out. Freedom To Marry has launched a $1 million satellite effort spearheaded by young conservatives who hope to expunge anti-gay language from the GOP platform, to “modernize” the party.

“We’re going to encourage voices within the party who support the freedom to marry to speak up and get the party where it needs to be — in alignment with a majority of the American people, of independents, and younger Republicans,” Freedom to Marry head Evan Wolfson tells me.

Yet the party platform still opposes gay marriage. A large majority of evangelical protestants still oppose it, and Mike Huckabee has warned that GOP support for it will cause the evangelical base to “take a walk.” Meanwhile, Cruz seems to be planning to make opposition to gay marriage a part of his case that the GOP can only win in 2016 by remaining faithful to pure conservatism — or his version of it, anyway.

An intra-GOP primary dispute over it could help reinforce a dynamic Ron Brownstein has already identified, one in which cultural issues such as gay rights have “reaffirmed the GOP’s identity as the champion of the forces most resistant to the profound demographic and cultural dynamics reshaping American life — and Democrats as the voice of those who most welcome these changes.” One can even imagine a viral moment  in which all the GOP candidates are asked to raise their hands if they believe marriage is only between a man and a woman. If Rob Portman doesn’t run, all the GOP candidates’ hands may promptly shoot heavenward.


UPDATE: And right on cue:

Utah Attorney General Sean Reyes will ask the Supreme Court to weigh in on a challenge to his state’s gay marriage law, he said Wednesday, potentially setting up Kitchen v. Herbert to be the case that legalizes gay marriage nationally.