If the democrats want to end the war they need to do nothing, because they have already ended the war. They sent a funding bill to Bush with some withdrawal language in it. Bush said he will veto it. Fine.
If he does veto the bill, he will have vetoed the funding for his war, and the war will have to be ended soon because there will be no money to pay for it. If he does not veto it, the bill mandates a beginning of the withdrawal, so the war will end in a little over a year.
That is where it stands right now. The only way to screw this up is for the Democrats to pass some lame war funding bill without withdrawal language. If the Democrats do not cave in and give Bush what he wants, the war cannot proceed.
The only way the war can proceed is if the Democrats do something. If they do nothing, the war will end. In effect, Bush needs the Congress to re-authorize the war.
John Edwards has pledged to not vote for a war funding bill without the withdrawal language, and he has challenged the other presidential candidates to follow suit. It will be interesting to see if the Democrats have the moxie to follow through with their plan to end the war by withholding a "clean" war funding bill.