President Obama has previously said that he does not support gay marriage — preferring instead civil unions that would provide similar legal protections — but that his position was "evolving."

On Monday, another member of Obama's Cabinet came out in support of gay marriage.

Arne DuncanArne Starkey DuncanBiden says 'enough is enough' after Santa Fe school shooting Obama Education secretary: Pull children out of schools until gun laws change Obama Education secretary mocks Pruitt over staff raises MORE, secretary of Education, was asked on MSNBC's "Morning Joe" if he believed gay marriage should be legal.

"Yes, I do," Duncan responded, saying he didn't know "if I've ever been asked" about his position on the subject before.

Asked about the vice president's comments on a conference call with reporters Monday morning, senior campaign adviser David Axelrod said the vice president's statement was "entirely consistent with the president's position."

Gay marriage proved a tough wedge issue for Democrats in the 2004 presidential campaign, when Republicans sought to drive evangelical voters to the polls with state-level anti-gay-marriage initiatives. And while national attitudes have shifted to favor gay marriage — a national poll from Pew released last month shows support for same-sex marriage edging opposition 47 percent to 43 percent — legalization remains unpopular among African-American and elderly voters.

In North Carolina, an important swing state and home to the Democrats' 2012 national convention, voters will head to the polls Tuesday to consider a constitutional amendment banning gay marriage. According to a PPP poll released Monday, the amendment seems likely to pass, with the legislation leading by a 55-39 percent margin among likely voters.