They could try if they're stupid enough to. It still doesn't change the fact that the Executive Branch is the one to deal with the UN and whatever the liberals in congress try to do won't have any effect on our policies.
Except regarding agreement to international treaties.
Well, yes, I agree that the Administration will stick to its guns and I also think that the Iraqi government will want the multi-national force to stay for the time being. However, what would be the effect of the UN not acceeding to an extension of the January 31, 2006 mandate (at the urging of Bush's opponents done without publicly acknowleging what they were doing, of course). My guess is that we will hear screaming about how our presence is now illegal just to create some mischief, which would then certainly be used back here to advance the dems position on withdrawal.