Well yes and no. First of all the administration should have KNOWN that libs would detract from the war from day 1. This should have been thrown into the pot when they went into this thing. But yes, if this thing is not won at this point, it won't be because of a few car bombs in Bagdad. It will be largely because of the political firestorm on the left.
By the way, in my honest opinion, this thing in Iraq is a done deal. Dems will hang themselves on this issue...I'd put money on it. but Bush has 3 years to tie up the loose ends though.