Webb, a Democrat, withdrew his plan shortly before noon after it fell four votes shy of the 60 needed to advance.

While the 56 votes it attracted are a majority in the 100-member Senate, Republican leaders used Senate rules to set a 60-vote requirement to end debate and bring the issue to a final vote.

Webb said he was trying to "establish a fair minimum floor for the use of American troops," some of whom are now on their third or fourth tour of duty in Iraq. His proposal would have required the administration to give active duty troops returning from the war zone "dwell time" at their home bases equal to the time they were deployed.

The Army currently sends forces into Iraq for 15-month tours of duty but guarantees them only a year between deployments. While troops return to the U.S. during those breaks, they are away from their families for additional months as they train for their return to the fighting.

Webb's plan would have given reservists and National Guardsmen breaks between deployments at least three times as long as their time away from home.

"We owe stability and a reasonable cycle of deployment to the men and women who are bearing this nation's burden" in Iraq, Webb asserted shortly before the vote.

Sen. John Warner, R-Va., was among seven Republicans to buck the party line and support Webb's proposal. He is among a growing group of GOP senators who appear increasingly uneasy with the administration's management of the war.