WASHINGTON – The Senate rejected a constitutional amendment to ban gay marriage by a wide margin yesterday, delivering a stinging defeat to President Bush and other Republicans who had hoped the issue would rally GOP voters for the November elections.

The senators’ vote was 49-48 to limit debate and bring the amendment to a yes-or-no decision.That was 11 short of the 60 needed, killing the measure in the Senate for this year.

Mr. Bush suggested the ban was proper and its time would still come. He said, “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.”

Democrats suggested it was all about conservative politics.

“Why is it when Republicans are all for reducing the federal government’s impact on people’s lives until it comes to these stinging litmus test issues, whether gay marriage or end of life, they suddenly want the federal government to intervene?” Senator Feinstein, a Democrat of California, asked. “It makes no sense other than throwing red meat to a certain constituency.”

The 49 votes to keep the amendment alive were one more than the measure received the last time the Senate voted, in 2004. Proponents had predicted the amendment would get at least a 51-vote majority in the 100-member Senate with the gain of four Republican seats since then.

It takes two-thirds majorities in both houses of Congress to send a proposed amendment to the states for ratification. The House will take up the issue next month.

Despite the defeat, amendment backers insisted progress had been made because the debate over three days raised the issue’s profile and will force candidates to answer for their votes on the campaign trail.

“Eventually, Congress is going to have to catch up to the wisdom of the American people or the American people will change Congress for the better,” Senator Vitter, a Republican of Louisiana, said.

“We’re not going to stop until marriage between a man and a woman is protected,” Senator Brownback, a Republican of Kansas, said.

Most bitter to the amendments’ authors was the loss of support in their own GOP caucus. Two Republicans changed their votes from yes in 2004 to no this time: Judd Gregg of New Hampshire and Arlen Specter of Pennsylvania. Chuck Hagel of Nebraska did not vote yesterday because he was traveling with Mr. Bush.

All told, seven Republicans voted to kill the amendment. The five others were Lincoln Chafee of Rhode Island, Susan Collins of Maine, John McCain of Arizona, Olympia Snowe of Maine, and John Sununu of New Hampshire.

Mr. Gregg said that in 2004, he believed a Massachusetts Supreme Court decision legalizing same-sex marriage in that state would undermine the authority of other states, like his, to prohibit such unions.

“Fortunately, such legal pandemonium has not ensued,” Mr. Gregg said. “The past two years have shown that federalism, not more federal laws, is a viable and preferable approach.”

A majority of Americans define marriage as a union of a man and a woman, as the proposed amendment does, according to a poll out this week by ABC News. But an equal majority oppose amending the Constitution over the issue, the poll found.

The tally put the ban 18 votes short of the 67 needed for the Senate to approve a constitutional amendment.

Supporters of the amendment acknowledged disappointment in the vote and, to some extent, Mr. Bush’s advocacy. “He could have done more, but he doesn’t have a vote in this one,” Mr. Brownback said of the president.

Forty-five of the 50 states have acted to define traditional marriage in ways that would ban same-sex marriage – 19 with state constitutional amendments and 26 with statutes.