I agree with Chris bowers:
Klein's central premise is that Progressives have no leverage to make Blue Dogs want to vote for good legislation, since opposing Democrats is popular in their districts. However, Blue Dogs have leverage over Progressives, since Progressives don't want Democrats to lose seats.
The reason I disagree with Klein is fairly simple: if no health care legislation passes, and Democrats lose seats as a result, Blue Dogs are the people who will lose the seats, not Progressives. Even if Klein is correct and Democrats lose a bunch of seats because Progressives blocked it, Blue Dogs are actually the ones who will bear the brunt of those losses. As such, Blue Dogs have more to lose if health care fails to pass than Progressives.
Read it all at Open Left