After a solid act one in the Defense Authorization fight, in which Senate Majority Leader Harry Reid actually made the Republicans follow through on their filibuster threat, he's pulled the bill entirely until the GOP allows a vote on the Reed-Levin amendment.
Reid's got absolutely nothing to lose here. With public approval of congress lower than even our approval of George Bush, Reid has a lot of incentive to make this a fight about acting on the will of the public. Particularly since Harry Reid and the Democratic Party are, you know, on the side of the public will.
And don't forget: the reason that the GOP doesn't want this amendment to move forward is that it would trigger a veto from their boss and lay the story out clearly for everyone to see: Democrats are working extraordinarily to force a course change in Iraq, and George Bush and his GOP are quashing every attempt to end their very unpopular war.