WASHINGTON — The Democratic Party is moving to include support for gay marriage in the official party platform for the first time, a Democratic official said Monday, marking a key milestone for advocates of same-sex unions.

The party’s platform drafting committee voted to include language backing gay marriage during a weekend meeting in Minneapolis, the official said. Democratic delegates will formally approve the platform during the party convention in Charlotte, N.C., in early September.

President Barack Obama will officially accept his party’s nomination at the convention, which marks the start of the fall campaign blitz. Republican rival Mitt Romney will get the GOP nomination a week earlier during his party’s convention in Tampa, Fla.

Seeking to ramp-up enthusiasm among Democrats, party officials said Sunday that former President Bill Clinton will deliver the nominating speech on Wednesday night of the convention. Obama and Vice President Joe Biden are to speak on Thursday, the convention’s final night.

The Obama campaign and convention organizers on Monday announced that Elizabeth Warren, the Democratic Senate candidate in Massachusetts and a popular figure among liberals, will get a featured speaking role. Warren headed the Congressional Oversight Panel for the Troubled Asset Relief Program, the committee that examined the bank bailout. She also was the leading advocate for a consumer financial protection agency, created under the 2010 overhaul of financial regulations. Warren’s convention address will precede Clinton’s.

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.

The Democratic official would not comment on the exact language of the pro-gay marriage plank approved by the drafting committee. It was unclear if the party would call for any national action to legalize gay marriage. The official spoke on the condition of anonymity because that person was not authorized to speak publically about the platform committee’s decision.

Obama has said he considers gay marriage to be a state issue, not a federal matter.

Gay rights advocates hailed the decision as a significant step forward.

“I believe that one day very soon the platforms of both major parties will include similar language,” said Chad Griffin, president of the Human Rights Campaign. “There is no more American value than honoring and protecting one’s family.”

The National Organization for Marriage, which opposes same-sex unions, said the decision sets up marriage as a defining issue in the presidential election.

“We will rally supporters of traditional marriage to make sure they realize that the outcome of the presidential election may determine the future of marriage in our country,” said Brian Brown, the organization’s president.

The Democratic platform committee’s move was first reported by The Washington Blade.