Hawai'i Sens. Daniel Inouye and Daniel Akaka, both Democrats, helped to block the amendment.

Akaka said: "The constitution I cherish and have sworn to uphold is not meant to be used as a sword against individuals or unpopular views. Rather, it is a shield intended to protect the rights of all citizens. ... I shudder to think of the tragic consequences for our nation if previous congresses decades ago had yielded to the passions of the day and attempted to amend the constitution in response to contentious court decisions. The proposed amendment seeks to limit the rights of certain individuals based on their sexual orientation. The states have the authority to address this issue; indeed the people of Hawai'i have spoken on this matter as have other states. I believe that such an amendment is unnecessary and, therefore, I oppose the resolution."

Bush issued a statement saying he was "deeply disappointed" with the vote.

"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 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.

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.

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.