However, this pattern of asymmetric polarization is not present in other estimates of ideology. Data from Adam Bonica‘s CFscores (“campaign finance” scores), which provide ideology estimates for members of congress based on donations to and from each member, indicate that since 1980 congressional Democrats have moved left slightly more than congressional Republicans have moved right, as shown below.
DW-Nominate scores are uncontroversial as proxies for legislator ideology within a given Congress, but use of DW-Nominate scores to make comparisons across time has drawn questions. DW-Nominate scores are centered over time based on assumptions about cross-time legislator ideological change, but CFscores and Bailey scores attempt to center ideal points across time with non-legislator bridge observations. Bridge observations for CFscores are donors and for Bailey scores are political positions. Examples of such bridges include interest group A donating to legislator B in 1984 and donating to legislator C in 1996 (for CF scores), or legislator D in 1980 and legislator E in 1990 announcing support for the Supreme Court’s decision in case F (for Bailey scores).
It is not that one technique is superior to the others. But one technique may be more useful for some purposes and the other technique more useful for other purposes. This decision, though, requires careful thought and a detailed understanding of the modeling assumptions of each technique. Although DW-Nominate scores suggest that recent polarization has been driven mainly by Republicans moving right, other estimates present a different pattern, complicating recent accounts and inviting reflection about how polarization is defined and measured.
L.J. Zigerell is an assistant professor of political science at Illinois State University.