US President George W Bush says he will carry on trying to make homosexual marriages illegal in the United States.
Some states and towns have gone ahead with gay weddings
Mr Bush was speaking after a constitutional amendment on the issue was rebuffed by the Senate.
The president is seeking to change the US constitution to specify that marriage can only take place between a man and a woman.
In a statement, he said he was deeply disappointed at what had happened, but described it as a temporary block.
"Activist judges and local officials in some
parts of the country are not letting up in their efforts to
redefine marriage for the rest of America - and neither
should defenders of traditional marriage flag in their
efforts," he said.
The Republican proposal failed to gain the support of the 60 senators it needed to progress to the next stage.
BBC religious affairs correspondent Jane Little says that to many Americans the constitution itself is sacred - despite 11,000 attempts, it has been amended only 17 times in 200 years.
Republicans vowed to fight on, but the measure is now unlikely to be passed before the November elections.
"I would argue that the future of our country hangs in
the balance, because the future of marriage hangs in the
balance," said Senator Rick Santorum.
But the Democrats' leader in the Senate, Tom Daschle, said there was no "urgent need" to amend the constitution.
Republicans say they have been forced to take action after Massachusetts' highest court ruled that gay couples could be legally wed, and the city of San Francisco started
issuing marriage certificates to gay partners.
There are 38 US states which have already banned homosexual marriage, but lawsuits in Florida, Nebraska, New Jersey and Oregon are seeking to have it ruled legal.
Mr Bush was hoping to make individual states' attempts to legalise it irrelevant, by outlawing it nationwide.
In his weekly radio address at the weekend, he called marriage between a man and a woman "the most
fundamental institution of civilisation".
"This difficult debate was forced upon our country by a few activist judges and local officials, who have taken it on themselves to change the meaning of marriage," he said.
"A constitutional amendment should never be undertaken lightly
- yet to defend marriage, our nation has no other choice."
His Democrat rival for the White House, Senator John Kerry, says that he and his running mate, John Edwards, think states should be allowed to look after the matter themselves.
"Both John and I believe firmly and absolutely that marriage is
between a man and a woman. But we also believe that you don't play
with the constitution of the United States for political purposes
and amend the Bill of Rights when you don't need to," he said.