CQ has an update on the behind-the-scenes debate over the Webb amendment:

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.

Alexander, Murkowski, and Specter had been thought of as potential supporters of the Webb amendment. I've heard from a source that Webb and supporters are concerned that Warner's alternative proposal--which would be a non-binding "sense of the Congress"--would be used as "cover" for wavering senators to avoid supporting the (requirement-backed) Webb amendment.

Debate over this Webb amendment seems to be on two levels: its constitutionality and its worth as a policy measure. Many opponents of the amendment think that it is unconstitutional for Congress to decide troop movements and also claim that it could lead to a "backdoor" withdrawal from Iraq--in addition to raising fears of a"management nightmare." Backers of the amendment argue that soldiers need more time to recover between tours of duty--and often launch into wider critiques of the administration's Iraq policy. Debate over this measure seems often to be really a debate about Iraq and our capacity for success there.

UPDATE: It looks like there may be a vote on Webb at 5:15 EST (or that's what leaders are planning at the moment).