A 2011-era FiveThirtyEight article (https://fivethirtyeight.com/features/how-liberal-is-presiden...) looked at average DW-NOMINATE scores over time for the House of Representatives. Its conclusion was:

> According to the system, both parties have been on a trajectory toward more “extreme” positions since roughly 1970, the natural result of which is more polarization. However, the parties do not quite share equal responsibility for this: Republicans have moved about twice as much to the right as Democrats have to the left. Also, while the Democrats’ leftward shift was essentially a one-off event, the result of many moderate, Southern Democrats losing their seats in the early 1990s, the Republicans’ rightward transition has been continuous and steady.

Although to caution too-credulous interpretation, the DW-NOMINATE scores are derived from roll-call votes, so they may be biased by the issues brought up for such votes.