If the Democrats were serious about ending the war [and obviously they're not], they would simply filibuster any military appropriations bill that didn't fund an immediate troop withdrawal. They would need only 40 votes to sustain the filibuster - and there would be nothing for Bush to veto.
I'm sure the Republicans would then start braying about "the nuclear option" again. [The Senate rules one - not the "make Tehran radioactive" one
] And who would prevail in that contest seems uncertain.
But that's what a serious anti-war position would look like.