The move, which is not final, would place the party in line with President Barack Obama's personal beliefs. On May 9 he became the first sitting president to say gay men and lesbians should be legally able to marry.
The undivided recommendation, made by a 15-member drafting committee in Minneapolis Sunday and first reported by the Washington Blade Monday, is widely expected to be approved, perhaps with a few adjustments, by the full platform committee when it meets in Detroit Aug. 10-12.
The approved document is then expected to be forwarded for approval by delegates to the party's national convention, to be held in Charlotte, N.C., the week of Sept. 3.
Democrats said Monday former President Bill Clinton would give a speech on the economy at the convention Sept. 5 and formally place Obama's name into nomination for re-election. Vice President Joe Biden is expected to speak the following day, introducing Obama before his acceptance speech.
The platform language approved Sunday reiterated the party's call for the repeal of the federal Defense of Marriage Act, which defines marriage as between a man and a woman, the Blade and other news organizations reported.
Gay rights supporters praised the Democratic Party vote, while conservative groups expressed dismay.
"Like Americans from all walks of life, the Democratic Party has recognized that committed and loving gay and lesbian couples deserve the right to have their relationships respected as equal under the law," Chad Griffin, president of the Human Rights Campaign, the largest rights-advocacy group for the lesbian, gay, bisexual, and transgender community, said in a statement.
"I believe that one day very soon the platforms of both major parties will include similar language on this issue," he said.
Peter S. Sprigg, a senior fellow at the Family Research Council, a conservative Christian group, predicted Democrats would regret the decision.
"There are many places in the country where Democratic candidates will not want to be identified with the gay-marriage party," he told The New York Times. "I think this is more politically correct than it is politically smart."
Tim Wildmon, president of the American Family Association, which promotes fundamentalist Christian values, told The Wall Street Journal he considered including the marriage plank in the Democrats' platform "terrible news."
"I'm a little surprised they would put it in the platform, because there are a lot of Democrats who don't agree with that, especially in the black community," he said.
The Rev. G. Modele Clarke, senior pastor of the predominantly black New Progressive Baptist Church in Kingston, N.Y., said historically black Christian denominations might have to shift their same-sex marriage position.
"The black church has been very conservative theologically," taking the position the Bible is the inerrant word of God, and considering marriage between a man and a woman, Clarke told the Kingston Daily Freeman.
But "there are times when the Bible upheld things like slavery, genocide, polygamy and other practices that would be indefensible today," he said.
"If somebody tried to defend slavery today based on biblical teachings and precepts, it wouldn't hold up," he said. "So when I look at this issue of same-sex marriage ... I'm thinking, Are we on the same path that we were on when we defended slavery?"