There is a lot of talk about political polarization in Congress. But is it true? Well, yes. Keith Poole and Howard Rosenthal, social scientists at the University of Houston and Princeton, respectively, used systematic measures of liberalism and conservatism built around government intervention in the economy to chart roll-call votes in Congress.

We have adapted their scores to look at the House and Senate in each decade from 1955 to the present. The result? Thirty-three percent of House members were near-pure centrists in 1955; in 2004, just over eight percent fit that category. Thirty-nine senators were centrists in 1955, compared with nine in 2004.

The differences are attributable to the emergence of the permanent campaign, the rise of partisan news media and, most of all, changes in Congressional redistricting. The expansion in the number of safe seats in the House that began in the 1980's has put an increased importance on primaries, which favor more ideological candidates. A number of these sharp-edged representatives have then moved to the Senate, where they have helped widen the partisan gulf we have talked about--and now can see.

Norman J. Ornstein is a resident scholar and Barry McMillion is the Ford Fellow in Political Studies at AEI.