Congress can't do anything to get the troops pulled out of Iraq besides cut any and all funding for the current operations and the Department of Defense, the problem with that option is that it abandons all troops in Iraq and leaves them with no way out while also leaving the military with no money to use to purchase the supplies needed to ensure the overall safety of the nation. The War Powers Act won't hold up if Congress tries to impose it because it infringes on the President's role as the Commander-in-Chief, he's called that because he is the highest authority. If Congress were at the top of the totem poll the President would simply be another link in the chain and not the C-in-C. So basically by making their goal "Getting out of Iraq" the Democrats gave themselves an impossible task and when they don't follow through on it, or really make a dent on it, they lose crucial votes. Plus there's the issue of the President as being seen as being responsible for defending the nation and almost all polls confirm that the American people do not have the same amount of confidence in the Democrats as they do the Republicans on issues of defense and national security. Which are kind of a big deal for the President.