He would do it by telling Bush that he would not receive any more funding to continue the war. Pelosi could do this right now, if she really opposed the war, by not allowing any more funding bills to be brought to the floor. The head of the budget committee could do the same thing by not allowing any funding bills to leave committee. That's how the Vietnam war was ended. Congress refused to fund it any longer. If we waited until presidents were willing to end wars, even Democratic ones, they would never end.