"Pressure vs Purge"
Ruth Marcus critiques a straw man version of the idea that there ought to be tighter party discipline:
One difficult test of who’s right here involves the role of the conservative House Democrats, Blue Dogs and others. White House Chief of Staff Rahm Emanuel helped elect them. Now he has to cope with the consequences of having a Democratic caucus with a cadre of members significantly to the right of their party but in the leftward precincts of their districts. If they vote like prototypical Democrats, they won’t be coming back.
The problem here is that it vastly underestimates the ideological flexibility available to an incumbent legislator. We know that a great deal of flexibility exists because each state features two senators with the exact same electorate. And yet Sherrod Brown and George Voinovich have very different voting records. So do Evan Bayh and Richard Lugar. So do Chuck Grassley and Tom Harkin. Where a member winds up positioning him or herself within that feasible ideological space is going to depend in part on his or her conscience and in part on the degree of electoral pressure he or she faces in the form of fear of primary challengers and the like. This is pretty commonsensical, and thinking that more pressure could be helpful by no means commits you to the obviously silly few that everyone should vote exactly like Jerry Nadler.