Democrats are moving, for the first time, to include support for same-sex marriage in the official party platform at their national convention.

The decision comes after President Obama became the first sitting U.S. president to voice support for gay marriage, and as party officials try to build the anticipation ahead of the convention set for early September in Charlotte, N.C.

Sources told Fox News that the Democrats' platform drafting committee voted unanimously to include the language backing same-sex marriage at a meeting in Minneapolis over the weekend. The next step would be for the full platform committee to take it up in Detroit next month. From there, the plank would go to convention delegates in Charlotte.

The drafting committee decision leaked out shortly after officials revealed that former President Bill Clinton will deliver the nominating speech on Wednesday night of the convention. Obama and Biden are to speak on Thursday, the convention's final night.

Several prominent Democrats began pushing earlier this year for support of same-sex marriage to be included in the convention platform, which lists principles the party supports. The effort got a boost in May when Obama voiced his personal support for same-sex unions.

New Hampshire Democratic Sen. Jeanne Shaheen hailed the decision in a statement Monday.

"Equality and freedom are at the core of who we are as Americans, and that means freedom for all of us, not just some of us. Our party has long been a leader on issues of justice and this reaffirms to the country that we are working on behalf of all our families, gay and straight," she said.

The president's support for same-sex marriage, though, continues to stir controversy. As reports first surfaced of the drafting committee's decision, an organization of black pastors announced Monday that they'll be launching a national campaign starting Tuesday in opposition to Obama's endorsement.

"President Obama has not given us the courtesy of a reply to discuss his support of same-sex marriage, and is taking the black vote for granted," Rev. William Owens, president of the Coalition of African American Pastors, said in a statement. He said the campaign will urge black pastors and other black voters to "withdraw their support" of the president.

"We will see that the black community is informed that the president is taking them for granted while pandering to the gay community," he said.

It's unclear how detailed the language is on the same-sex marriage plank, and whether it calls for any legislative efforts like the repeal of the Defense of Marriage Act -- which defines marriages on the federal level as between a man and a woman.

Fox News' Mike Emanuel and The Associated Press contributed to this report.