Jon Chait nails it.
From the link:
The psychology of victory and defeat is a remarkable thing. A week ago, the Democrats were perceived to have an enormous political problem. Their agenda was stalled in Congress. There was a mass groundswell of public anger they had to contend with.
Suddenly those problems have been flipped on their head. Now Democrats don’t have a problem because they can’t pass anything, Republicans have a problem because they’re obstructing everything. Whereas right-wing grassroots activism represented a public backlash against the Democrats, it’s now seen as an extremist element that discredits the GOP. Political reporters are starting to construct a seamless narrative connecting the over-the-top rhetoric from GOP and conservative leaders, the unusual acts of obstructionism and legislative retribution (like canceling unrelated hearings as revenge for health care reform), and sporadic vandalism and threats of violence. For example, see Dana Milbank’s column today.