In this article from the New York Times, the writers talk about how Democrats have kept their Congressional unity over Iraq even in the face of Bush's tough-guy talk. The fact that Democratic majorities in both the House and the Senate have stayed together on the Iraq vote is remarkable. In the past Bush could intimidate enough Democrats from "red" states or Congressional districts to give his policies a veneer of "bi-partisanship".
This intimidation took place because Democrats from such areas were afraid of his political power. Since he is now polling in the low to mid-thirties on job approval, his political power has diminished. The diminishment of Bush's political power combined with the power of being the majority party has emboldened Democrats and led to party unity on Iraq.
If Democrats keep united, they will force Bush to respond to them because it is hard to conduct a war without bi-partisan support for that war. Increasingly Republicans from "blue" states or Congressional districts will be under pressure to desert Bush on Iraq. It will be interesting to see how long Republicans will keep supporting Bush' war as we get closer to the 2008 elections.