The markup comes shortly after President Obama came out in support of same-sex marriage last week, following increasing calls from top Democrats for Obama to add the issue to his reelection platform. He previously was against same-sex marriage but was pro-civil union and, in 2011, directed the Justice Department to stop defending the Defense of Marriage Act.

Even if the legislation does pass the Senate, it will likely die in the Republican-controlled House.

In early May, Vice President Biden came out in support of same-sex marriage during an appearance on NBC's "Meet the Press."

"I am absolutely comfortable with the fact that men marrying men, women marrying women and heterosexual men marrying women are entitled to the same exact rights. All the civil rights, all the civil liberties," Biden said.

Same-sex marriage will likely be a dominating topic on the campaign trail as the presidential election approaches. Shortly after Biden's comments and a day before Obama said he supports same-sex marriage, North Carolina, which Democrats hope to win in 2012 and the state in which they are hosting their 2012 national convention, passed legislation banning same-sex marriage.