WASHINGTON (CNN) -- A measure that would have forced the Pentagon to give troops sent to Iraq stateside leave equal to their time in the battle zone was defeated Wednesday evening in the Senate after failing to draw enough Republican votes.
Sen. Jim Webb, D-Virginia, right, and Sen. Chuck Hagel, R-Nebraska, talk Wednesday about troop rotations.
The amendment from Sen. Jim Webb, D-Virginia, won the support of 56 senators, four short of the 60 votes needed under Senate rules to move it forward. A similar measure had failed to gain approval in July.
Webb's amendment to a defense authorization bill had been met by a veto threat from the Bush administration, which argued the amendment would hamstring the Pentagon's ability to deploy troops as needed.
The measure's chances of passage took a blow earlier in the day when influential GOP Sen. John Warner of Virginia, who had voted for it in July, announced he would no longer support it.
"I agree with the principles [in the amendment], but I regret to say I've been convinced by those in the professional uniform that they cannot do it and do it in a way that wouldn't invoke further unfairness to other soldiers now serving in Iraq," Warner said on the Senate floor Wednesday afternoon.
Webb's measure got 56 votes in July, and Warner was one of seven Republicans who crossed party lines to support it.
In August, Warner was openly critical of President Bush's war strategy, calling for 5,000 troops to come home by Christmas.
The six Republicans who broke ranks to vote for Webb's amendment were Norm Coleman of Minnesota, Susan Collins and Olympia Snowe of Maine, Gordon Smith of Oregon, Chuck Hagel of Nebraska and John Sununu of New Hampshire.
On September 13, the president announced he will bring home about 5,700 troops by the end of the year. Defense Secretary Robert Gates has said the Webb amendment would tie the hands of the Pentagon and hurt the war effort.
Democrats were looking at plans other than Webb's.
Senate Majority Leader Harry Reid said Tuesday he plans to push an amendment offered by Sens. Carl Levin, D-Michigan, and Jack Reed, D-Rhode Island, that would set "definite timelines" for bringing home American combat forces.
Reid said the Levin-Reed proposal is "basically the same as it was" in July, when it drew the support of 52 senators, including four Republicans.
The Senate could start debate on Iraq this week, and, despite the impasse in negotiations over Levin-Reed, Democratic leadership aides said they are not ruling out compromises on other Iraq-related measures that could attract 60 votes.
Sens. Ken Salazar, D-Colorado, and Lamar Alexander, R-Tennessee, are working to refine and update their legislation to codify recommendations of the Iraq Study Group. And Sen. George Voinovich, R-Ohio, unveiled a proposal Tuesday to change the mission for U.S. troops in Iraq, but without setting a deadline for it to happen.
Some GOP senators, including Arizona Sen. John McCain, said Webb's proposal amounted to an unconstitutional legislative infringement on Bush's powers as commander-in-chief -- an argument Reid flatly rejected Tuesday. Watch Webb call for more downtime for the troops »
"We have a responsibility, by virtue of the Constitution, to work with the military, to set policy for the military," he said. "Anyone that suggests that the Webb amendment is unconstitutional either is not reading the law, or no one's explained it to them very well." E-mail to a friend
CNN's Dana Bash, Lisa Goddard and Ted Barrett contributed to this report.