WASHINGTON — Republican leaders vowed Wednesday to keep fighting for a constitutional amendment to ban same-sex marriage, even though they failed to win even a simple majority on a fatal procedural vote and were far from the two-thirds majority eventually needed to pass the amendment.

"The battle is just beginning," said Sen. Wayne Allard, R-Colo., main sponsor of the amendment, after losing 48-50 and likely killing the amendment for the year.

That total was 12 votes short of the 60, or three-fifths majority, needed to cut off debate and vote on the amendment itself. It was 19 short of the 67, or two-thirds majority, needed to pass a constitutional amendment. Such amendments must pass by two-thirds majorities in both houses and be ratified by three-fourths of state legislatures.

Even though it was a procedural vote, Sen. Bob Bennett, R-Utah, one of the GOP's top vote counters as its chief deputy whip, said the totals "give a pretty clear picture of where we are (in support margins) for the amendment itself."

On the losing side were 45 Republicans and three Democrats. On the winning side were 43 Democrats, six Republicans and one independent. Sens. John Kerry, D-Mass., and John Edwards, D-N.C., skipped the vote while campaigning for president and vice president but have said they oppose the amendment.

They and most Democrats indicate they personally oppose same-sex marriage but say the matter should be left to the states. They say the federal Defense of Marriage Act passed years ago is sufficient to guarantee that no state need recognize same-sex marriages performed in other states.

While amendment supporters obviously have far to go, "It doesn't mean it won't be back. This is a serious matter. . . . It's like the Civil Rights Act was defeated at first, and they brought it back year after year, gaining support until they finally passed it," Bennett said.

Bennett said Republicans sought the ability in the debate to alter wording of the amendment — possibly to make it more clear that civil unions for gays would be allowed — to gain votes. Democrats, however, vowed to filibuster, or talk to death, anything but a straight up-or-down vote on Allard's version of it.

Bennett said that doomed the GOP's ability to gain more votes, and it is why Republican leaders went to the procedural vote to try to thwart the threatened Democratic filibustering.