Nor is Snyder alone among swing-state GOP governors in declining to oppose the issue's advance. In Nevada, Governor Brian Sandoval announced last month that he would no longer defend the state’s gay-marriage ban. In New Mexico, Governor Susana Martinez said in January she accepted gay marriage as "the law of the land."

Three GOP U.S. senators now support gay marriage, and 10 Republicans voted for the Employment Non-Discrimination Act in the Senate last year. (The measure, which would ban discrimination on the basis of sexual orientation by employers, is stalled in the House of Representatives.) In Illinois, gay marriage passed the state legislature last year when three Republican state representatives joined Democrats to support it. Social conservatives vowed electoral revenge, but in the state's Republican primaries last week, all three survived.

"Gay marriage is the last frontier of civil rights, and it would be nice if Republicans didn't have to be dragged kicking and screaming" toward acceptance, Mark McKinnon, a GOP consultant who once made campaign ads for George W. Bush, told me in an email. "The good news is, I think Republicans are coming around." National GOP politicians seem loath to broach the issue, and at gatherings like this month's Conservative Political Action Conference it gets less and less attention, he noted: "The wedge has lost its edge."

A decade ago, Republicans campaigned aggressively on their opposition to gay marriage. Bush promised to push for a constitutional amendment to ban it; the state-level gay-marriage amendments that appeared on 11 states' 2004 ballots passed with an average of 70 percent of the vote, helping turn out Bush voters in the process. Fast forward to today: Bush's onetime campaign manager, Ken Mehlman, came out in The Atlantic in 2010 and is now a strategist and fundraiser for gay marriage nationally. More than 100 prominent Republicans signed onto a Supreme Court brief in support of gay marriage last year; 20 Western GOPers, including former Wyoming Senator Alan Simpson, endorsed a similar appellate-court brief this month. The brief was authored by a Denver lawyer who had served as counsel to Mitt Romney's campaign in Colorado, and who told the New York Times it was about "liberty and freedom."

It's not just lawmakers and consultants—it's voters, too. A New York Times/CBS News poll last month found 40 percent of Republicans support gay marriage, up from just 24 percent in September 2012. Young Republicans are partly responsible: A Pew poll this month found 61 percent of Republicans under 30 support gay marriage. But that doesn’t suffice to explain the phenomenon. “We’re witnessing more than just generational turnover,” Tyler Deaton, campaign manager for Young Conservatives for the Freedom to Marry, wrote in a memo on the GOP shift scheduled to be released Wednesday. “Conservatives are changing their minds.”