The modern two-party system in the US Senate

I’ve created a timeline of the ebb-and-flow of party politics in the US Senate since the beginning of the modern (Democrat & Republican-dominated) two-party era. Beginning with the antebellum 35th Congress, and progressing through to the 109th, this timeline tells the story of the evolution of politics in America as played out on the floor of the Senate.

Political Scientists, Historians, and even casual observers of political history have long noted the shift in ideological nature of the two major parties since the Civil War, and within the 20th Century alone–this timeline conveys a sense of that shift by depicting the scaled left-right ideological positions of each Senator along the vertical axis: a macro view of the entire time period illustrates the great distance between the parties up until the mid-20th Century, at which time Civil Rights-related issues began to create crosscutting cleavages within the parties, especially in the South. The bright green of Southern Democrats, voting with the Republicans in a Conservative Coalition, is readily apparent, as the distance between party medians converges.

Just as apparent is the realignment beginning in the late 1970s-early 1980s, where, as Political Scientists have documented, the polarization of the electorate and of elected officials became a dominant trend. This is illustrated both in the main timeline, and also by the series of density plots just below the main frame. At various times, the ideological distribution is obviously unimodal, or obviously bimodal: from the 79th to the 109th Congress we can witness the polarizing of the Senate, which according to the theory of Conditional Party Government, has lead to skewed policy outcomes.

This visualization rewards careful inspection–there are stories to be found everywhere: follow the positions of the Presidents, relative to their own party members; note how party leaders are typically close to their own party median, find patterns in individual states’ ideology over time–at any level of inspection, the data offer up a rational yet compelling history.

The ideological dimensions are determined based on Senate Roll-Call votes, and scaled to be historically consistent, such that comparisons may be made across historical eras. The measure is called DW-NOMINATE, and was developed by Kieth Poole and Howard Rosenthal. Please leave any comments or questions, as this work is constantly under progress.