They said their strategy would now focus on portraying Republicans as opposing any change and on trying to chip away support for the White House as the war continued.

The proposal that failed Wednesday fell 4 votes short of the 60 needed to prevent a filibuster and would have required that troops be given as much time at home as they had spent overseas before being redeployed.

There were 56 votes in favor, including 6 Republicans - one fewer than the 7 Republicans who joined the Democrats in July, when the measure, by Senator Jim Webb, Democrat of Virginia, also fell 4 votes short.

Supporters of Mr. Bush's war strategy declared victory, saying they had firmly beaten back legislative efforts to change course.

"It means that Congress will not intervene in the foreseeable future," said Senator Joseph I. Lieberman of Connecticut, the Independent who has voted with the Republicans on war issues. "The fact that it didn't get enough votes says that Congress doesn't have the votes to stop this strategy of success from going forward."

The Senate vote was a crucial test of the war plan that Mr. Bush put forward last week, calling for only gradual reductions in troop levels in Iraq from their current high, and leaving intact by next summer a main body of more than 130,000 troops, about the same number as last February.

The outcome showed that the strong opposition to the war plan by Democrats and a few Republicans remained insufficient to overcome a powerful Republican minority in the Senate that has succeeded all year in staving off challenges to the war policy.

For now, the failed Webb proposal is the closest Democrats have come to bipartisan legislation that would force Mr. Bush to change his strategy. And with Republicans solidly behind the plan outlined by Mr. Bush and Gen. David H. Petraeus, the commander in Iraq, Democrats have retreated to a firm antiwar stance.

They are no longer entertaining the kind of compromise measures that some Democrats had proposed this month as an attempt to woo Republican defectors, and they said they would instead seek opportunities to hold votes that would more starkly contrast Republican support for the president with Democrats' demands for withdrawal.

"The Republican leadership and the White House is getting them all to march in line," said Senator Charles E. Schumer, Democrat of New York, who ranks third in the party leadership. "But it is marching further and further away from where America is. We just keep at it. It's all we can do."

Democratic strategists and party officials said that Senator Harry Reid of Nevada, the majority leader, and his colleagues decided to stop trying to strike a deal with Republicans after they found little interest on the other side and could not settle on a plan that would appeal to Republicans but was tough enough to hold Democrats together.

Jim Manley, a spokesman for Mr. Reid, said the majority leader was rebuffed repeatedly in his efforts to find consensus with the Republicans.

"It became evident that Republicans were not willing to break with the president," he said.

Democrats said Mr. Webb's proposal, if approved, would have added time between deployments, forced the withdrawal of troops on a substantially swifter timeline and, they said, protect troops from serving protracted and debilitating deployments.

On Thursday and Friday, the Senate is expected to vote on several other war proposals by the Democrats, including one by Senator Russ Feingold, Democrat of Wisconsin, that would require most American troops to be pulled out of Iraq by next June and would then cut financing for continuing military operations.

Another proposal by Senator Carl Levin of Michigan, chairman of the Armed Services Committee, would require a shift of American troops away from combat by next summer. Mr. Reid's spokesman said the decision to stick with a hard deadline for withdrawal was endorsed by Mr. Levin, who earlier had signaled a willingness to soften his proposal to win Republican converts.

Neither the Feingold plan nor the Levin initiative has much chance of winning 60 votes.

The Senate will also vote on a plan by Senator Joseph R. Biden Jr., Democrat of Delaware and chairman of the Foreign Relations Committee, that calls for a greater reliance on diplomacy to forge a political solution in Iraq and end the war.

But Democrats seemed resigned to having little chance of influencing the war strategy anytime soon.

After the vote, a dejected Mr. Webb said: "You are seeing, as of a week ago, the administration and some of the leading Republicans in here talking about, 'Hey it's O.K. that we're going to be in Iraq for the next 50 years.' I don't think it is O.K."

He continued: "And so we are going to have this debate. It is going to be a long and emotional debate, long meaning in months and perhaps years."

Mr. Webb's plan came under sharp attack by the Pentagon, which said it would interfere with complex troop deployment schedules, and late last week the administration put intense pressure on Republican lawmakers when it became clear that Mr. Webb was close to securing enough Republican votes to win.

And it was dealt a death blow when Senator John W. Warner, Mr. Webb's fellow Virginian, and one of the most respected Republican voices on military affairs, announced dramatically on the Senate floor that he was withdrawing his support for the proposal based on information provided by Defense Secretary Robert M. Gates and other senior military officials.

"I endorsed it," Mr. Warner said of Mr. Webb's plan. "I intend now to cast a vote against it."

In explaining his decision, Mr. Warner said he had been convinced, at a meeting earlier in the day with senior military officials, that the Webb plan would cause havoc for the armed forces, potentially lengthening tours in Iraq. He also met with Mr. Gates, a longtime friend, on Monday.

But Mr. Warner's change in position echoed a wider unwillingness by Republicans to break ranks with the administration.

The two Republican senators running for president - John McCain of Arizona and Sam Brownback of Kansas - voted against the Webb proposal. Four Democratic candidates - Hillary Rodham Clinton of New York, Barack Obama of Illinois, Mr. Biden and Christopher J. Dodd of Connecticut - voted for it.

The continuing partisan divide frustrated some moderate lawmakers in each party who are eager to help shape Iraq policy and signal to constituents that they are working to end the war.

Mr. Reid traveled on Monday to New York City to help raise money for antiwar groups, and while Democrats remain under substantial pressure from those groups, Mr. Manley and others said that event was scheduled weeks ago and had no bearing on the legislative change of tack.

Senator Lindsey Graham, Republican of South Carolina, who worked to defeat the Webb plan, said the Republican support for the war could have a political cost. "The Republicans own this war," he said. "If it goes bad, the nation loses and the Republican Party loses disproportionately compared to the Democratic Party."