Washington ? President Bush and supporters of a constitutional amendment banning gay marriage suffered an embarrassing defeat Wednesday when the proposal failed to get a simple majority in the Republican-controlled Senate.

The amendment lost with 50 senators voting to block it and 48 voting to advance it on a procedural vote, leaving backers 19 votes shy of the two-thirds majority they eventually would need to pass a constitutional amendment saying marriage can be only between one man and one woman.

“The president and the Republican leadership’s attempt to divide the country backfired and instead it divided the Republican Party,” said Cheryl Jacques, the president of Human Rights Campaign, a gay rights organization that opposed the amendment.

Six Republicans bucked their party leadership and voted against the measure. Three Democrats sided with Republicans on the procedural vote, which needed the support of 60 senators to bring the amendment to a full Senate vote.

Supporters of the amendment said they weren’t embarrassed, proclaiming Wednesday’s vote a positive first step. They said they were happy to highlight the gay marriage issue during more than three days of debate.

“The battle has just begun,” said Sen. Wayne Allard, R-Colo., a co-sponsor of the amendment.

Conservative activists said they would continue to press the issue and that Wednesday’s vote showed them which senators they needed to try to defeat in November elections.

“There ought to be votes until the desire of the American people has been realized on this,” said Gary Bauer, the president of the conservative group American Values, citing national polls that show a large majority of Americans oppose marriage for gays and lesbians.

Bush said he was “deeply disappointed” by 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,” Bush said in a written statement. He urged the House of Representatives to approve the amendment, which it will consider in September.

But the Republican decision to focus on gay marriage could harm the party with moderate voters this November and put some moderate members of Congress in an awkward situation, said Sherry Bebitch Jeffe, a political analyst at the University of Southern California.

“It was not a success on any level for the president,” she said. After a Massachusetts Supreme Court ruling last year that legalized gay marriage and with San Francisco issuing same-sex marriage licenses, Bush in February called for a constitutional amendment banning such unions. He devoted his weekly radio address Saturday to the issue.

Sen. John Kerry, D-Mass., the Democratic presidential candidate, and his running mate, North Carolina Sen. John Edwards, oppose gay marriage but don’t believe a constitutional amendment is necessary. They didn’t cast a procedural vote on Wednesday.

Democrats this week charged that Bush and Senate Republicans were deflecting attention from ignoring more important issues, such as terrorism and U.S. intelligence failures, by focusing on gay marriage.

“This is merely a political issue,” Sen. Christopher Dodd, D-Conn., said Wednesday, noting that no federal court has ruled in favor of same-sex marriages. “It’s a waste of our time.”

But Republican supporters said “activist judges” were preparing to strike down the federal Defense of Marriage Act, a 1996 law signed by President Clinton that prohibits same-sex marriages. They said that waiting until federal courts act would be too late.

Supporters of the amendment were careful in their arguments not to criticize gays and lesbians, reflecting the potential political ramifications of the debate in an election year.

“It is not about hate. It is not about gay bashing,” said Sen. Rick Santorum, R-Pa. “It is simply about doing the right thing for the basic glue that holds society together.”

But Democrats said the goal of amendment supporters was to limit the rights of gays and lesbians and to enshrine discrimination into the Constitution.

“Gays and lesbians across America look to the Senate to see if we will brand them as inferiors in our society,” said Sen. Patrick Leahy, D-Vt. “I will not diminish other Americans in the Constitution.”