Democrats are poised to include support for same-sex marriage in the party's platform of official policy positions when they meet at the national convention in September, sources told ABC News.

A committee of Democrats drafting the platform unanimously approved a same-sex marriage plank during their meeting in Minneapolis this weekend. It's the first time the party as a whole has formally expressed support for the unions.

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The language endorsing same-sex marriage must still be approved by a larger committee of Democrats tasked with finalizing the platform before being put to convention delegates for final approval, sources familiar with the process said.

The move follows public calls by several high-profile Democrats, including House Minority Leader Nancy Pelosi and Senate Majority Leader Harry Reid, to include marriage rights for gays and lesbians as an official policy position.

In June, President Obama reversed his longstanding opposition to same-sex marriage in an interview with ABC News, a shift that many advocates say cleared the way for inclusion of the issue in the Democratic Party platform.

Supporters of same-sex marriage hailed the pending move by Democrats as a reflection of a broader trend in American society.

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

Opponents of gay unions said the Democrats' platform would energize conservatives to turn out to vote.

"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, president of the National Organization for Marriage.

This post has been updated.