Forty-eight senators voted to advance the measure — 12 short of the 60 needed — and 50 voted to block it. Defeat came at the hands of dozens of Democrats joined by six Republicans.
Bush issued a statement saying he was "deeply disappointed" with the vote, but casting it as a temporary setback. "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.
Senate Democratic leader Tom Daschle said there was no urgent need to amend the Constitution. "In South Dakota, we've never had a single same sex marriage and we won't have any," he said shortly before the vote. "It's prohibited by South Dakota law as it is now in 38 other states. There is no confusion. There is no ambiguity."
The amendment provided that marriage within the United States "shall consist only of a man and a woman." It also required that neither the U.S. Constitution nor any state constitution "shall be construed to require that marriage or the legal incidents thereof be conferred upon any union other than the union of a man and a woman."
The Senate acted as House Republicans began advancing legislation that would bar federal courts from ordering states to recognize same-sex marriages sanctioned in other states. The measure cleared committee on a vote of 21-13 and is expected on the House floor next week.
GOP leaders also have discussed granting Bush his wish to have the entire House vote on the proposed constitutional amendment in the fall, even though the Senate's rejection effectively killed any chance it could be submitted to the states for ratification this year.
Whatever the legislative endgame, supporters said they expected the issue to play a role in political campaigns.
"I think it will be a significant issue in the fall elections as it is further engaged," said Sen. Sam Brownback, R-Kan.
"Four million religious conservative voters sat out the last election, so the president's visible stance on protecting marriage is essential to turning out all of those conservative voters who pulled the lever for him in 2000 and getting those other 4 million to come out for him this year," said Keith Appell, a conservative strategist in Washington.
"We now know which senators are for traditional marriage and which ones are not, and by November, so will voters in every state," promised Tony Perkins, president of the Family Research Council, which supports the amendment. He said nine states are poised to have state constitutional amendments on their ballots this fall on marriage, adding, "This fight has just begun."
Cheryl Jacques, president of the Human Rights Campaign, a gay rights organization, professed a lack of concern. "I think the discussion will continue to play out but I think they played their best hand today and couldn't even get a simple majority," she said.
Bush's Democratic rival, Sen. John Kerry of Massachusetts, skipped the vote. He issued a statement expressing renewed opposition to the amendment and accusing Republicans of seeking to alter the Constitution for political gain. "The unfortunate result is that the important work of the American people — funding our homeland security needs, creating new and better jobs, and raising the minimum wage — is not getting done," he said.
The issue also has flared in congressional campaigns in recent days as the Senate plunged into debate.
Former Housing Secretary Mel Martinez, in a multi-candidate battle for the GOP senatorial nomination in Florida, began running a radio ad last Friday urging the state's two Democratic senators to support the amendment.
Daschle, in a tough race for re-election, faces pressure on the issue as well. His opponent, former Rep. John Thune, began airing a radio commercial last week stressing his opposition to the amendment.
The issue will play a role in Senate campaigns "to the extent that there are differences" between the Republican and Democratic candidates, predicted Sen. George Allen, R-Va., chairman of the GOP senatorial campaign committee.
Polls show that while gay marriage is opposed by a strong majority of Americans, opinion is more evenly divided on the question of amending the Constitution. Republican strategists concede they must be careful in their handling of the issue, lest the GOP appear intolerant and offend moderate voters.
Sen. Rick Santorum, R-Pa., complained more than once during the Senate's debate that supporters of the amendment had been disparaged as intolerant.
"I would argue that the future of our country hangs in the balance because the future of marriage hangs in the balance," he said shortly before the vote. "Isn't that the ultimate homeland security, standing up and defending marriage?"
But critics declined to yield ground, calling the amendment an effort to shift attention away from the economy and the war in Iraq. Echoing Kerry, Sen. Christopher Dodd, D-Conn., said, "The issue is not ripe. It is not needed. It's a waste of our time. We should be dealing with other issues."
In all, 45 Republicans and three Democrats voted to advance the measure, while 43 Democrats, six Republicans and one independent voted to scuttle it. In part, the GOP defections reflected a concern that the Senate should not pre-empt what has historically been a state's rights issue.