Like Paul, I'm convinced that any candidate who doesn't support marriage equality will instantly be disqualified as a plausible Democratic presidential nominee following Obama. Acceptance for same-sex marriage is growing rapidly across all ideological divides, and is particularly pronounced among liberals. In an alternative reality where the Democrats had an open primary in 2012, Obama's "evolving" stance on same-sex marriage would no longer pass muster in the Democratic base.
Obama's former opponent and current secretary of state Hillary Clinton has already shifted her views, supporting marriage equality when it was up for debate in New York. And just look at the language of the up-and-coming leaders of the Democratic Party. Two of the leading 2016 possibilities—Andrew Cuomo and Martin O'Malley—are governors who staked out legalized marriage equality as their major accomplishment.
Now another politician bandied about as a future Democratic leader is attacking Obama's wishy-washy defense of LGBT rights. “I want to see the president evolve because I believe that is right; marriage equality is morally right,” Elizabeth Warren said during an interview with the Washington Blade. Warren also attacked the Defense of Marriage Act in the interview and said she hopes Democrats make ending the discriminatory law a central tenant of the party platform. “Any steps that the president can take toward non-discrimination benefit the whole country,” Warren said. “I don’t know how else to say it. It’s the right thing to do.”
There's a possible fight brewing for the Democratic convention in September over whether the party adds support for marriage equality to the platform. It's an insider's argument; no one outside the activists or the media are aware of the contents of a party's platform. It'll be hard to add such a measure when the top of the ticket opposes same-sex marriage, though Obama is increasingly out of sync, not only with liberal activists, but the other leaders of his party.