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. Bush to change his strategy. And with Republicans solidly behind the plan outlined by 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.”