Interviewed on NPR a couple of days ago, Congressman Jack Murtha stated that Democrats would not withhold funding as a means of pressuring the White House to redeploy U.S. troops out of Iraq. The troops should not be put at (further) risk, he said, as part of a political struggle. Are Murtha and other Democrats too hastily rejecting this option? Can defunding be used to pressure the President without jeopardizing the troops? Maybe.
Congress clearly has the power to attach conditions on funding. Thus, it could pass funding legislation that says something like this: "None of these dollars shall be used for any purpose other than the expeditious withdrawal of U.S. forces from Iraq." But it would face two difficulties: (1) How to enforce such a condition? Presumably the courts would try to avoid adjudicating an inter-branch conflict between congressional power of the purse and the President's role as Commander in Chief. Congress might create an office of auditor within the military, but this just shifts the enforcement problem. Suppose the President now resists the auditor's authority. In any event, these ideas are probably beside the point because of difficulty number (2) Congress could only get such a measure enacted with 2/3 majorities in each house, because the President would presumably veto the bill. That won't happen.
Nonetheless, a bare majority in Congress does still have power here, even if it approves some funding for the war without strings attached: Namely, Congress can refuse to fund other military and/or civilian programs unless the President agrees to its terms. It could, in other words, dare the President to shut down the government. The last time that happened, President Clinton bested Speaker Gingrich in the court of public opinion, as most people blamed Congress rather than the President for the shutdown. But it hardly follows that President Bush would best Speaker Pelosi in a new game of chicken. It's all in the spin, and Democrats could plausibly claim that they have a mandate for redeployment from the election so that Bush, not Pelosi, could be portrayed as the intransigent one. I suppose the best one can hope for is that all of this is unnecessary, that with Rumsfeld's resignation and James Baker's committee likely to recommend some form of redeployment, a confrontation can be avoided.