As everyone knows, the Democratic Party has become more liberal, and the Republican Party more conservative over the past, oh, thirty years, with the latter effect dominating. But how did this happen? Why did it happen? Here's a fascinating research project, being conducted by Sean Theriault, of the University of Texas, that takes a magnifying glass to Congress over the past thirty years, examining over each seat in detail. Congress' hard jerk to the right, Theriault finds, owes mostly to the replacement of southern Democrats with Republicans than anything else.

That shouldn't make such a big difference, one would think, but it does: the southern Democrats of old tended to have reasonably liberal positions on most matters, with the big exception of racial and social issues. (One should note, though, that southern Democrats often blocked progressive legislation, such as health and labor law, for racial reasons—so in practice this distinction sometimes didn't matter.) Modern-day Republicans in the south, however, trend very conservative. This difference really can't be overstated. When Democrats have replaced Republicans in the northeast and other liberal areas, the ideological shift often turned out to be much smaller.

The second cause of Congress' rightward lunge, meanwhile, is the replacement of Republicans with other Republicans. That, of course, means the replacement of moderate Republicans with more conservative Republicans. In the House, since most races are so uncompetitive, the primaries are where all the action takes place. The Club for Growth or Christian Coalition or some other "rally the base" group will often fund a hard-right challenger for a sitting Republican they deem insufficiently radical. And the challenger doesn't even need to win to have an effect. If I remember correctly, political scientists have estimated that a serious primary challenge can pull a candidate quite a few notches closer to the ideological extremes. As it happens, Theriault found that these sorts of primary challenges happened far more among Republicans than Democrats.

Meanwhile, Theriault discovered that Republican politicians tend to move rightward once elected to office. Democrats sometimes drift, but not nearly as much. In fact, the bulk of the Democratic Party's "drift" to the left can be explained by the loss of their southern base. That's about it. Well, that and the fact that since 1950, the Democrats have increasingly represented more and more of the poor and working-class. (That is, class matters more for party affiliation now than it did half a century ago.) But honestly, that fact matters somewhat less for polarization, since both parties still cater to their donors, that is, those making over $100,000.