Sean M. Theriault, "Party Polarization in the US Congress: Member Replacement and Member Adaptation," Party Politics, 12 (July, 2006), 483-503.

First Paragraph:
The verbal assaults between the political parties in the United States now mirror the war between good and evil in professional wrestling. The antics in Congress over the past few years include three-hour roll-call votes, discussion of the 'nuclear option', charges of 'fruitcake' and the transformation of the Ways and Means library into a fortress by the panel's Democrats. As a bevy of media outlets dissect, analyze and denounce these 'legislative' maneuvers, the 'legislators' engage in their own form of hyperbole, threats and verbal flexing. Indeed, Hulk Hogan would be proud.

Figures and Tables:
Figure 1. Party polarization in the US House, 93rd to 108th Congresses (1973-2004)
Figure 2. Party polarization in the US Senate, 93rd to 108th Congresses (1973-2004)
Table 1. Polarization scores for Kentucky members in the 104th Congress
Table 2. Polarization by Congress for both the House and Senate, 93rd to 108th Congresses (1973-2004)
Table 3. Adaptation in the Senate and House, 93rd to 108th Congresses (1973-2004)
Table 4. Legislators with the highest adaptation scores
Table 5. Replacement in the Senate, 93rd to 108th Congresses (1973-2004)
Table 6. Replacement in the House, 93rd to 108th Congresses (1973-2004)

First Paragraph of Conclusion:
The analysis in this article analyzes the micro-level changes in members' ideological scores to appreciate the macro-level divergence of the political parties in Congress since the 1970s. In so doing, I add evidence to confirm the first consensus reached by the polarization studies. The political parties in Congress are, indeed, more polarized now than they were in the early 1970s. I find evidence, however, that contradicts the second consensus. Without a doubt, the replacement of moderate members by more ideological members has driven the parties apart. But so, too, has member adaptation, even including those legislators who adapt to more moderate ideologies. More than one-third of the polarization in both the House and the Senate results from the adaptation of members to the ideological poles. As both parties move closer and closer to their ideological homes, Republicans in the Senate have moved three times faster than Democrats, and their House colleagues have moved just under twice as fast as House Democrats.