An amendment to mandate minimum rest times for U.S. troops between deployments faced long odds Wednesday after an influential Republican senator switched his position and said he would vote against it.

John W. Warner, R-Va., a senior Armed Services Committee member who voted for the amendment by Jim Webb, D-Va., when it last came to the floor in July, said he had changed his mind. He said the ranks of military specialists are so thin in many positions that commanders on the ground would be seriously hobbled by mandatory “dwell times” between deployments.

Bush administration officials have been furiously lobbying moderate Republican senators to oppose the measure.

The Webb amendment would require military personnel to be given at least as much time at home as they spend deployed to Iraq and Afghanistan. National Guard and reserve forces would have to be allowed three years at home for each one at war.

As the Senate debated the measure, the operations chiefs of the Joint Chiefs of Staff and the Army huddled with a small group of centrist Republicans in a Russell Senate Office Building room.

The group included Republicans John W. Warner of Virginia, Lisa Murkowski of Alaska, Lamar Alexander of Tennessee and Arlen Specter of Pennsylvania. Afterwards, several sounded as if they had been won over.

“My goal is not to create a management nightmare for our commanders,” said Alexander.

Warner announced on the floor that he would oppose the measure. He was one of 56 senators who voted for an earlier version in July, leaving Webb four votes short of the 60 needed to overcome a GOP filibuster.

In the face of another filibuster, several additional Republicans had been weighing a vote change to support the new proposal. But Warner’s switch the other direction is likely to bolster the administration’s case against it.