It's The Structure, Stupid

Jerome Armstrong: "What's more, though the notion of bipartisanship as exemplified by folks like Sam Nunn and Unity08 may sound laudable, as I've written at length before the bipartisanship of the 1970s and 1980s was a byproduct of the changing partisan leanings of the electorate coupled with decades-long Democratic dominance on the congressional level, two conditions we do not see today."

I'm not sure I agree with that in all the details, but it really is remarkable that for all the bellyaching about the decline of bipartisan behavior in DC there's very little attention paid to the fact that there are actual reasons this has happened beyond Newt Gingrich being a meany and bloggers being too shrill. The Jim Crow South gave rise to an odd structure of American political institutions whereby both of the parties contained substantial ideological diversity. This had the benefit of setting the stage for a wide array of cross-cutting alliances. It came, however, at the cost of consigning a substantial portion of the population to life under a brutal system of apartheid ruthlessly upheld through systematic violence.

After that system collapsed, there was a two decade or so period during which the voters and parties were re-aligning themselves during which we had cross-cutting alliances but no apartheid. And now the aligning process is done, so we have two parties where essentially all Democrats are to the left of essentially all Republicans and so you have relatively few genuinely bipartisan coalitions.