The Republican-controlled Senate 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 Wednesday -- to pass the amendment itself.

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.

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.

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.

-- Helen Dewar

Singer Pat Boone, with Sen. Rick Santorum (R-Pa.), speaks in favor of the amendment. The proposal fell 12 votes short of the 60 needed to move the legislation forward.