Senate Republicans blocked a plan on Wednesday to give U.S. troops in Iraq more home leave, defeating a proposal widely seen as the Democrats' best near-term chance to change President George W. Bush's Iraq strategy.

The measure to give troops as much rest time at home as they spent on their most recent tour overseas needed 60 votes to pass in the Democratic-controlled Senate; it received just 56 votes, with 44 against.

It had been offered by Sen. Jim Webb, a decorated Vietnam veteran and former Navy secretary. The Democrat said U.S. troops are being "burned out" by repeated redeployments to Iraq, with tours of up to 15 months and less than a year off in between.

The plan was strongly opposed by the Bush administration -- Pentagon Secretary Robert Gates called it a backdoor attempt to pull troops off the battlefield in the unpopular Iraq war, and said he would recommend Bush veto it if it passed. A similar bill has passed the House.

Democrats have struggled all year to pass legislation winding down the Iraq war, and they have other proposals waiting in the wings, including some that explicitly require combat troop pullouts. But these are considered even less likely than Webb's to get any time soon the 60 votes often required to advance under Senate rules.

Senate Majority Leader Harry Reid, a Nevada Democrat, vowed not to give up. "We on this side of the aisle are not going to stop waging the hard but necessary fight to responsibly end this war," he said before the vote, adding that Republicans were "more interested in protecting their president than protecting the troops."

After the Webb measure failed, Republicans proposed a non-binding "sense of the Senate" supporting the goal of the Webb plan if operational conditions permit. But it also failed to get the 60 votes required, after Webb announced he opposed it and asked others to vote against it.

After appearing to gain momentum in recent days, Webb's proposal won the votes of just six of Bush's fellow Republicans on Wednesday. That was one Republican less than Webb's plan persuaded in a previous vote in July.

The senator who switched was Webb's fellow Virginian John Warner, also a former Navy secretary and an influential Republican voice on military matters. He announced on Wednesday he could not support the Webb plan again because it could extend tours of duty for units already in the war zone.

"I agree with the principles that you have laid down in your amendment," the 80-year-old Warner said during debate.

"But I regret to say that 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."

Arizona Republican Sen. John McCain, a presidential contender, said the rejection of the plan was related to U.S. Iraq commander General David Petraeus' and Ambassador Ryan Crocker's testimony to Congress last week and their "sound argument that we are succeeding" in Iraq.

McCain argued that Webb's plan was unconstitutional. "We have one commander in chief and one only."

Fellow Republican Sen. Chuck Hagel of Nebraska, a co-sponsor of Webb's plan, said Republican senators were heavily lobbied by the administration against the measure.

"The White House has been very effective of making this a loyalty test for the party," Hagel said after the vote.

Webb had tried to make the proposal acceptable to the Bush administration, including a presidential waiver in operational emergencies and a 120-day enactment period so the Pentagon could make needed adjustments.

(Additional reporting by Richard Cowan)

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