In this post we use DW-NOMINATE scores to examine historical patterns in the ideological distribution of the parties in Congress. Specifically, we sort Republican and Democrats in the House and Senate from most liberal to most conservative, then mark the position of the members in the 10th and 90th percentiles. For example, a Democrat in the 10th percentile would be one who is more moderate than 90% of her party colleagues, or more liberal than only 10% of the party. Likewise, a Democrat in the 90th percentile would be one who is more liberal than 90% of his co-partisans. These figures give us an idea of how party coalitions in Congress are ideologically distributed over time.
The first figure below shows the House, followed by the Senate. The y or vertical axis represents position on the liberal-conservative scale, using the metric of DW-NOMINATE scores. In both graphs, it is clear that the most moderate Democrats and Republicans in both chambers have become more liberal and conservative, respectively, over the last 30 years. As noted in a previous post, this shift has been more dramatic among Republicans than Democrats. Prior to this there was considerable overlap between the 10th percentiles of the parties, meaning that at least 10% of Democrats were more conservative than the 10th percentile Republican, and vice versa. This overlap has entirely dissipated.