Reid Wilson for National Journal's On The Trail blog:

... After decades of avoiding what was once a toxic issue, Democrats are on the verge of establishing a new litmus test, one that will demonstrate just how quickly and dramatically the political landscape will change: When a new crop of Democrats run for the White House in 2016, the top contenders will likely all embrace same-sex marriage the way Republicans embrace a pro-life stand.

Just as there is little room for a pro-choice Republican in national politics, it's becoming increasingly likely that President Obama will be the last Democrat to win his party's presidential nomination without supporting same-sex marriage.

Several of the Democrats who might run in 2016 are already to the left of him on gay marriage. New York Gov. Andrew Cuomo helped shepherd a bill through the Republican-controlled state Senate in his first year on the job. Maryland Gov. Martin O'Malley led the push to pass similar legislation; he'll sign the bill next week.

... Meanwhile, there are indications that opposing same-sex marriage is as strict a litmus test for Republicans as supporting unions is for Democrats...

But for an issue that some Democrats believe cost them the 2004 election, it's shocking just how much public attitudes surrounding gay marriage have changed; eight years after Kerry lost, most up-and-coming leaders within the Democratic Party have fully embraced same-sex marriage. As one Democratic strategist put it, it's hard to think of another civil rights paradigm that has shifted so quickly, in any country, at any point in history.