WASHINGTON – The Senate blocked legislation Wednesday that would have regulated the amount of time troops spent in combat, a blow for Democrats struggling to challenge President Bush’s Iraq policies.

The 56-44 vote was four votes short of reaching the 60 needed to cut off debate. It was the second time in as many months that the bill, sponsored by Sen. Jim Webb, D-Va., was sidetracked. In July, a similar measure fell four votes short of advancing.

Failure of the bill was a sound defeat for Democrats, who have been unable to pass significant anti-war legislation by a veto-proof majority since taking control of Congress in January. Webb’s measure was seen as having the best chance at attracting the 60 votes needed to pass because of its pro-military premise.

The bill attracted three dozen co-sponsors, including Republicans Chuck Hagel of Nebraska, Olympia Snowe of Maine and Gordon Smith of Oregon. In July, a similar measure fell just shy of the 60 needed to advance and additional Republicans said they were considering it.

Snowe and Sen. Susan Collins, R-Maine, voted in favor of the measure.

But momentum behind the bill stalled Wednesday after Sen. John Warner, R-Va., announced he decided the consequences would be disastrous. Warner, a former longtime chairman of the Armed Services Committee, had voted in favor of the measure in July but said he changed his mind after talking to senior military officials.

The bill would have required that troops spend as much time at home training with their units as they spend deployed in Iraq or Afghanistan. Members of the National Guard or Reserve would be guaranteed three years at home before being sent back.

Most Army soldiers now spend about 15 months in combat with 12 months home.

“I think it’s very important that we just put a safety net under our troops,” said Webb, a former Vietnam veteran and Navy secretary.

Without more Republican support, Democrats are unlikely to pass other war-related measures.

In coming days, the Senate plans to vote on legislation by Sen. Carl Levin, D-Mich., that would order combat troops home in nine months. Levin, chairman of the Armed Services Committee, said his bill would allow some troops to remain behind to conduct such missions as counterterrorism and training the Iraqis; he estimated the legislation, if enacted, would cut troop levels in Iraq by more than half.

The Senate also plans to vote on legislation by Sen. Russ Feingold, D-Wis., and Majority Leader Harry Reid, D-Nev., that would cut off funding for combat next year.

The firm deadlines reflect a shift in strategy for Democrats, who had been pursuing a bipartisan compromise on war legislation. But after last week’s testimony by Gen. David Petraeus, the top military commander in Iraq, Democrats calculated not enough Republicans were willing to break party ranks and support more tempered legislation calling for combat to end next summer.

Sen. John McCain, the top Republican on the Armed Services Committee and a Vietnam veteran, said Webb’s bill was a “backdoor method” by Democrats to force troop withdrawals.

“We have a new strategy. We have success on the ground,” said McCain, R-Ariz. Pulling out troops would spark “chaos and genocide in the region, and we will be back,” he said.

McCain offered an alternative resolution that would identify equal deployment and training times as a goal, but would not mandate deployment restrictions. The resolution was aimed at peeling off Republican support and lessening the prospects of passage for Webb’s bill.

Defense Secretary Robert Gates said he would have recommended that President Bush veto Webb’s legislation if it is passed. The bill could force the military to extend tours, rely more heavily on reservists, or not replace units right away, even if they are needed, Gates said.

Webb and his supporters say the bill provides flexibility to avoid those pitfalls, including a presidential waiver if Bush can certify to Congress that ignoring the limitation was necessary to national security.

Webb amended the bill, after consultation with Gates, to exempt special operations forces and give the military 120 days to comply.