The growing generational divide over same-sex marriage is a looming problem for Republican candidates. Right now polls show 70% of Americans under the age of 35 favor same-sex marriage, while only 39% of those over the age of 55 do.

Real Clear Politics' Michael Barone puts a fine point on it:

The problem for Republican politicians is not that opposition to same-sex marriage antagonizes gay voters. According to exit polls in the last three presidential elections, gays and lesbians made up just 3 percent of the electorate, and they were one of the few groups that voted for John McCain in 2008 in larger numbers than had voted for George W. Bush in 2004.

The Republicans' problem is young voters. Huge majorities of them favor same-sex marriage, and for most of them it's simply a no-brainer. They must have been turned off if they were watching the Republican presidential candidates vie with each other in opposing it in the Fox News-Washington Examiner debate in Iowa.

The constitutional amendment banning same-sex marriage that they supported is never going to get a two-thirds vote in Congress or be ratified by three-quarters of state legislatures. Unless the Supreme Court rules there is a constitutional right to same-sex marriage, this is an issue that is going to be decided by the states.