The Republican-controlled Senate yesterday blocked a proposed constitutional amendment to bar same-sex marriage, effectively killing the White House-backed measure for the rest of this year and handing President Bush a big election-year defeat.

The vote was 48 to 50 against bringing the initiative to a vote, 12 short of the 60 needed to limit debate and move toward final action on the amendment. It would have taken a two-thirds majority -- 19 votes more than the GOP had yesterday -- to pass the amendment itself.

Bush had pushed hard for the amendment, but, with six Republicans joining nearly all Democrats in blocking the initiative, the vote appeared to cast doubt over how well the marriage issue is likely to play in the president's reelection campaign and many of the closest congressional races.

Recent opinion polls show that, while a large majority of Americans oppose gay marriage, many oppose amending the Constitution to outlaw the practice, which became a national issue after Massachusetts's highest court legalized same-sex marriages and San Francisco began performing them.

Reflecting the polls, senators expressed reluctance to alter the Constitution to include a divisive social issue, especially at the expense of tampering with traditional state prerogatives over marriage law. Some also said they think current laws adequately protect marriage and expressed concern that the amendment would be interpreted as anti-gay.

In addition, some Republicans as well as Democrats questioned whether the amendment -- despite denials by its sponsors -- might cast legal doubt on civil unions that created some legal protections for same-sex couples. In addition to defining marriage as a "union of a man and a woman," the proposed amendment says that neither the federal nor state constitutions shall be interpreted to "require that marriage or the legal incidents thereof be conferred upon any union other than the union of a man and a woman."

Despite divisions in their caucus, Senate Republican leaders vowed to keep pushing for approval of the amendment, arguing that traditional marriage is in jeopardy from "activist" judges and hoping the issue will energize conservatives to help elect more supporters of the measure in this fall's elections.

"This issue is not going away," said Majority Leader Bill Frist (R-Tenn.). Sen. Jeff Sessions (R-Ala.) said, "We will be back again and again" and will eventually prevail.

In a statement, Bush said he was "deeply disappointed" by the Senate 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. Bush urged the House to pass the amendment.

Despite the fact that the Senate vote effectively kills the amendment because it must win support of both houses, the House plans to vote on a similar constitutional proposal in September. It also plans to act next week on a bill to strip federal courts of the power to review state laws that ban same-sex marriage. The court-stripping bill was approved, 21 to 13, by the House Judiciary Committee yesterday. Its chances in the Senate are regarded as dim at best.

Democratic presidential candidate John F. Kerry appeared as pleased as Bush was disappointed by the Senate vote. "The floor of the United States Senate should only be used for the common good, not issues designed to divide us for political purposes," the Massachusetts senator said in a statement issued by his campaign. He said the Senate should be spending its time on issues such as homeland security, job creation and raising the minimum wage.

Both Kerry and his running mate, Sen. John Edwards (N.C.), decided against coming back for the procedural showdown. Both said they oppose gay marriage but would have voted against the amendment.

Just as Republican leaders refused to concede defeat, advocacy groups that championed the amendment and jammed Senate telephone lines for three days described the vote as a "first step" toward eventual approval of the proposal. "Our amendment will continue to gain ground so long as activists continue to strike down our marriage laws in court," said Matt Daniels, president of the Alliance for Marriage, a coalition of civil rights, religious and other leaders who favor the amendment.

But the Human Rights Campaign, the nation's largest gay rights group, called the vote a defeat "for the politics of distraction." Republicans wanted to exploit the issue for political advantage but found that it backfired by creating divisions in their own ranks, said Cheryl Jacques, president of the group. "Every poll shows the American people want Congress focused on issues like rising health care costs, the hemorrhaging of jobs and the war in Iraq," not gay marriage, she added.

The four-day debate on the amendment ended as it began, on a sharply partisan note, with Republicans contending that the institution of marriage would be in jeopardy if opened to gays, and Democrats accusing Republicans of using a divisive issue to mobilize their conservative base.

"It's not about how to protect the sanctity of marriage," said Sen. Edward M. Kennedy (D-Mass.). "It's about politics -- an attempt to drive a wedge between one group of citizens and the rest of the country, solely for political advantage."

"No one wants to discriminate against gays," responded Judiciary Committee Chairman Orrin G. Hatch (R-Utah). "Simply put, we want to preserve traditional marriage."

Republicans "propose turning the Constitution of the United States from the fundamental charter preserving our freedoms into a kiosk for political bumper stickers," said Sen. Patrick J. Leahy (Vt.), ranking Democrat on the judiciary panel.

Not so, said Sen. John Cornyn (R-Tex.), a member of the committee. The real question is whether senators "believe traditional marriage is important enough that it deserves full legal protection."

Sen. John McCain (Ariz.), one of the six Republicans who voted to block the amendment, said he did not believe it would be approved "this year, nor next year, nor anytime soon until a substantial majority of Americans are persuaded that such a consequential action is as vitally important and necessary as the proponents feel it is today."

McCain was joined by Republicans Susan Collins (Maine), Olympia J. Snowe (Maine), Lincoln D. Chafee (R.I.), John E. Sununu (N.H.) and Ben Nighthorse Campbell (Colo.) in voting to block the amendment. Three Democrats voted to limit debate: Robert C. Byrd (W.Va.), Ben Nelson (Neb.) and Zell Miller (Ga.). Virginia GOP Sens. John W. Warner and George Allen voted to limit debate, while Maryland Democrats Paul S. Sarbanes and Barbara A. Mikulski voted against it.

Staff writer Charles Babington contributed to this report.

Amendment backers Sens. Wayne Allard (R-Colo.), right, John Cornyn (R-Tex.) and Sam Brownback (R-Kan.), far left, are expected to keep trying to ban gay marriage. Sen. Patrick J. Leahy (D-Vt.), with Reps. Diana DeGette (D-Colo.) and Joseph Crowley (D-N.Y.), rear, said after the vote yesterday that supporters of the amendment to ban gay marriage were attempting to turn the U.S. Constitution into "a kiosk for political bumper stickers."