( CNN ) -- Efforts of Senate Democrats to get U.S. combat troops out of Iraq were damaged Wednesday when Virginia Republican Sen. John Warner said he would not back a plan to mandate stateside leave time
"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.
Democrats had hoped Warner would back an amendment from Virginia Democratic Sen. Jim Webb that would mandate that troops are given stateside leave equal to their time in the battle zone.
Webb's measure got 56 votes when it was brought up during the summer, and Warner was one of seven Republicans who crossed party lines to support it, Webb said.
Earlier Wednesday, Webb said, "I'm still hopeful that Senator Warner will support this, will vote for it."
In August, Warner was openly critical of President Bush's war strategy, calling for 5,000 troops to come home by Christmas.
In the past week, the president has announced he will bring home about 5,700 troops by the end of the year. And Defense Secretary Robert Gates has said the Webb amendment would tie the hands of the Pentagon and hurt the war effort.
Senate Democrats need 60 votes to avoid a Republican filibuster and get any plan to a vote. Democrats were hoping support from the influential Warner would swing other GOP senators their way.
It would take 67 votes to override a presidential veto.
Democrats were still looking at plans besides 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 insist 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.
Sen. Arlen Specter, another Republican who admits he's undecided on an Iraq withdrawal plan, said he was to meet Wednesday with Warner and Pentagon officials.
"I want to know what is going to happen with the force structure ... in Iraq," Specter said.
The Bush administration strongly opposes Webb's amendment, and Gates said he would urge Bush to veto it.
Also, some GOP senators, including Arizona Sen. John McCain, maintain that Webb's proposal amounts to an unconstitutional legislative infringement on Bush's powers as commander-in-chief -- an argument Reid flatly rejected Tuesday.
"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.