In the discussions of the current government shutdown, many have mentioned the death of moderation in the Republican Party, with a revanchist minority taking the reins of the party and its direction. This piqued my interest in looking at the ideological evolution of both parties and the asymmetric polarization that has defined the past few decades.
To get a look at asymmetric polarization in action, I decided to pore through the data on voteview.com, which runs the DW-Nominate scoring system. The system provides a rank ordering of all of the members of the Senate and House in each Congressional session. Scores range from -1.000 (most liberal) to 1.000 (most conservative). You can read their methodology on the site linked above.
As is the case with any such scoring system, there are flaws. The most obvious and annoying one, in my opinion, is the fact that congresspersons are given only lifetime scores, so you can't see the evolution of a particular person's ideology. This also makes the rank-orderings of past Congresses imperfect because they are not treated as sessions frozen in time. The flaws of DW Nominate are perhaps most evident in the scoring of Kirsten Gillibrand (D-NY). Gillibrand went from being a Blue Dog representative from upstate New York to a member of the liberal faction of the Senate Democrats, now representing the full state. DW Nominate does not account for this because it only provides a single score based on her full career.
Nevertheless, the scoring system is still quite useful for tracking trends over time.
Out of curiosity, I decided to track how many representatives of each party received a score more than 0.500 or more than 0.600 away from the center. Democrats with scores between -0.500 and -1.000 can be called liberal Democrats, and Republicans with scores between 0.500 and 1.000 can be called conservative Republicans. The use of 0.600 adds another layer of intensity.
This allows us to see how strong the liberal faction of the Democratic Party and the conservative faction of the Republican Party are.
I decided to look at three Congressional sessions: the 100th (1987-1989), 106th (1999-2001), and 112th (2011-2013). The 113th session, the current one, has not been scored yet. This takes us from the end of the Reagan presidency to the end of the Clinton presidency to the present.
I tried to work with only the members currently seated at the end of the Congress so that I would not have any duplicates.
The 100th Congress had 254 Democrats and 178 Republicans at its end.
15 Democrats (5.9% of the caucus) had scores between -0.600 and -1.000.
42 Democrats (16.5% of the caucus) had scores between -0.500 and -1.000.
7 Republicans (3.9% of the caucus) had scores between 0.600 and 1.000.
26 Republicans (14.6% of the caucus) had scores between 0.500 and 1.000.
The 106th Congress had 210 Democrats and 222 Republicans at its end.
16 Democrats (7.6% of the caucus) had scores between -0.600 and -1.000.
54 Democrats (25.7% of the caucus) had scores between -0.500 and -1.000.
24 Republicans (10.8% of the caucus) had scores between 0.600 and 1.000.
59 Republicans (26.6% of the caucus) had scores between 0.500 and 1.000.
The 112th Congress had 191 Democrats and 240 Republicans at its end.
18 Democrats (9.4% of the caucus) had scores between -0.600 and -1.000.
51 Democrats (26.7% of the caucus) had scores between -0.500 and -1.000.
57 Republicans (23.8% of the caucus) had scores between 0.600 and 1.000.
113 Republicans (47.1% of the caucus) had scores between 0.500 and 1.000.
Across Congresses, the size of the liberal faction has been relatively the same; its share mainly fluctuates because of the size of the total caucus. The movement to the left of the mean DW Nominate score of the Democratic Party (which you can see on the website) is not a result of a stronger liberal faction, but rather a result of the disappearance of conservative Southern Democrats from the caucus.
The conservative faction of the Republican Party has seen substantial growth, both in terms of raw numbers and in terms of its share of the party. Conservative Republicans now make up almost half of the caucus.