Same-sex marriage is the most dramatic political shift of the current era. In 2004, referenda to ban it were so popular that they were used to drive up turnout; by 2013, most Americans favored marriage equality. But that rolling tide of change has crashed to a halt in Alabama. Even though a federal judge has ruled that the state cannot continue to bar same-sex couples from getting married, the state's chief justice has advised probate-court judges not to issue licenses, and most are complying.

This pose threatens to provoke a constitutional crisis, with states attempting to defy federal authority. We've seen this play out before—the states lost, but it sometimes took some serious federal muscle to force their compliance.

If this was going to happen, though, it's perhaps not surprising that this would happen in Alabama, home of George Wallace. The state is last in the nation in support for marriage equality—tied with next-door neighbor Mississippi. Although majorities of Americans now favor same-sex marriage, just 32 percent of Alabamians and Mississippians do, according to numbers from the Public Religion Research Institute. PRRI is releasing an unusually large data set, including 40,000 interviews conducted over 2014 in all 50 states, as part of the American Values Atlas. (See the full numbers at the bottom.)

The biggest factor in the low support seems to be the high concentration of white evangelical Protestants in those two states—39 percent of Alabamians identify as members of that demographic, double the nationwide average. Even as the rest of the South has undergone huge shifts in opinion toward marriage equality, Alabama and Mississippi remain outliers.

While Alabama's opposition marks it as an outlier, the state actually appears to be on the same trajectory as the rest of the nation, just not as far along the curve.

Dig into the details and you can see how. For example, 48 percent of Alabamians under 35 favor gay marriage—not a majority, but a plurality. (Only 21 percent of those older than 65 do.) The strongest predictor for supporting gay marriage nationwide, said PRRI CEO Robert P. Jones, is having close gay friends, which is far more common among younger Americans. It's a larger factor than whether respondents are Republicans or Democrats. Younger Alabamians are far less likely to identify as white evangelical Protestants (just 24 percent) and almost twice as likely to be religiously unaffiliated, as their fellow citizens (19 versus 11 percent).

"This pattern is absolutely consistent across the country," Jones said. "Younger Americans, whether they come from Alabama or Delaware, are more likely to support same-sex marriage than their parents and grandparents. They're much more like to have a close friend or family member who's gay or lesbian. They are much less likely to be religiously affiliated at all."

Alabama is unusual in that slightly more black than white respondents favor gay marriage, by a margin of 23 percent to 22. Nationally, the opposite is true—only 41 percent of black Americans in PRRI's survey back marriage equality.

Could the nationwide trend reverse itself, and could gay marriage become significantly less popular around the nation? Though a pending Supreme Court case could make the constitutional question irrelevant, the politics are, at least in theory, more changeable. But Jones doubts that the pendulum will swing back.

"If in fact these were rooted only to theological beliefs or only to political beliefs, we might expect the winds to shift and maybe they might go in a different direction," he said. "Because these numbers are really rooted for younger Americans in the social fabric of friendship, it makes them much more stable."