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 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.

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.

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.

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

Sen. John McCain of Arizona, 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.