The US Senate has blocked a constitutional amendment banning same-sex marriage that had the backing of President George W Bush.
Some US states fully recognise same-sex marriages
The motion was backed by 49 to 48 senators, well short of the number needed to go to a final vote.
Mr Bush said he was disappointed but called it "the start of a new chapter in this important national debate".
Critics say Mr Bush is trying to win back disillusioned Republican voters ahead of November's mid-term poll.
The decision to throw out the ban was not a surprise and broadly reflects a national consensus, the BBC's James Coomarasamy in Washington says.
Although a majority of Americans oppose same sex marriage, most want individual states to make their own decisions about the issue, our correspondent adds.
But supporters of the amendment, including Mr Bush, said they would not give up their fight.
"Our nation's founders set a high bar for amending our constitution - and history has shown us that it can take several tries before an amendment builds the two-thirds support it needs in both houses of Congress," Mr Bush said.
"My position on this issue is clear: marriage is the most fundamental institution of our society, and it should not be redefined by activist judges."
Massachusetts Democrat Senator Edward Kennedy accused the Republican leadership of "asking us to spend time writing bigotry into the constitution".
"A vote for it is a vote against civil unions, against domestic partnership, against all other efforts for states to treat gays and lesbians fairly under the law," he said.
In a floor debate on Tuesday, he called it a "cynical attempt to score political points by overriding state courts and intruding into individuals' private lives".
The subject of gay marriages has been much debated in the US since Massachusetts became the first state to issue marriage licences for gay couples in 2004.
Forty-five of 50 states have passed laws or amended their constitutions to effectively prohibit same-sex marriages.
But several of those bans have recently been rejected by judges in states, including Washington, California and New York.
Although a similar ban on gay marriages failed to pass through Congress in 2004, it proved an issue that energised socially-conservative Republicans in that year's presidential poll, our correspondent says.
A constitutional amendment needs two-thirds support in both the House and the Senate.
It would then be sent to the individual states for ratification, and would need to be approved by three-fourths of them.