WASHINGTON - The City Council voted yesterday to legalize gay marriage, giving supporters a victory after a string of recent defeats elsewhere and sending the issue to Congress, which has final say over laws in the nation's capital.

Mayor Adrian Fenty has promised to sign the bill, which passed, 11-2. Gay couples could begin marrying as early as March if Congress allows it to become law. Congress has 30 days to act on the bill.

Democratic congressional leaders have suggested that they are reluctant to get involved, but gay-marriage opponents say they will try to get the measure overturned in Congress or at the polls.

Federal lawmakers declined to weigh in the last time they had a chance, after the council voted in May to recognize gay marriages performed elsewhere. Congress let that bill become law without taking any action, avoiding what would have amounted to a referendum on gay marriage.

The bill that passed yesterday had overwhelming support among council members, and the outcome was no surprise. Two council members said "I do" when their names came up during the vote.

When the vote was finished, a packed chamber erupted into cheers and clapping. "Make no mistake, 2009 has been one hell of a year for marriage equality," said David Catania, who introduced the bill and is one of two openly gay council members.

The "no" votes included former Mayor Marion Barry, now a council member, who voted, "I don't."

If the bill becomes law, the district will join Connecticut, Iowa, Massachusetts, and Vermont in issuing marriage licenses to same-sex couples. They will be able to wed in New Hampshire starting in January.

Gay-marriage supporters have had less success elsewhere recently. Maine voters overturned the state's same-sex marriage law last month. Earlier this month, the New York Senate rejected a bill that would have allowed gay couples to marry. And New Jersey's Legislature, which had been working on a same-sex marriage bill, postponed a recent vote when the measure appeared headed for defeat.

Yesterday's vote in the district came after several months of discussion, including two marathon council hearings at which some 250 witnesses testified.

Opponents included the Archdiocese of Washington, which said it might have to stop providing adoptions and other services because the law would force it to extend benefits to same-sex couples. The archdiocese is against gay marriage.

The law will likely take effect around St. Patrick's Day in this city of 600,000. Congress has 30 working days to reject it, but on previous matters, that has happened just three times in the last 25 years.

Still, opponents plan to try. Members of a group called Stand4Marriage, led by a local pastor, Bishop Harry Jackson, have met with members of Congress to urge them to oppose the bill.

Attorney Cleta Mitchell said that after Fenty signs the bill and it moves on to Congress, the group will ask a district elections board to put a referendum on the ballot asking voters to overturn the measure. She said in a statement before the vote that the law was a "decision for the people, not a dozen people at City Hall."

The group Mitchell represents made a similar request in the summer, when the city passed a law recognizing gay marriages legally performed in other states. The elections board declined to put the issue on the ballot, saying that would violate a city human-rights law.

Jackson said yesterday that he believed the group had an "airtight legal case" and that "if it gets to the vote, we win."

The group also has a lawsuit pending from earlier this year, when it tried to get an initiative on the ballot asking voters to define marriage as between a man and a woman. The elections board again cited the human-rights law in rejecting that request. A hearing in that case is scheduled for January.