Party leaders don't have the tools they once had to build cohesion in their parties. External forces are gaining strength, which tempers the argument that parties are the dominant institution within Congress.
Key bit from the interview in the story I linked to above:
What we're seeing is the collapse of institutional Republican power. It’s not so much about Boehner. It’s things like the end of earmarks. They move away from Tom DeLay and they think they're improving the House, but now they have nothing to offer their members. The outside groups don't always move votes directly but they create an atmosphere of fear among the members. And so many of these members now live in the conservative world of talk radio and tea party conventions and Fox News invitations. And so the conservative strategy of the moment, no matter how unrealistic it might be, catches fire. The members begin to believe they can achieve things in divided government that most objective observers would believe is impossible. Leaders are dealing with these expectations that wouldn't exist in a normal environment.
