Donald Trump will be sworn in as President of the United States next Friday, 20 January 2017. This surprised most analysts including ourselves! We expected to be writing posts during 2017 about splits in the Republican Party due to President Hillary Clinton’s pressing a legislative program that would have been attractive to enough Republicans to have caused serious conflict in the Republican caucuses. Instead we have a President who is both a real estate tycoon and a television entertainer. What his policy views are in many areas are opaque (to say the least).

What we find alarming is the unprecedented collapse of the long-term structure of Congressional Voting during the past 20 years. Contrary to what many scholars say when they cite our book, Congress: A Political-Economic History of Roll Call Voting (1997, New York: Oxford University Press), Poole and Rosenthal DO NOT CLAIM that voting in Congress is largely one-dimensional. Rather Poole and Rosenthal show that a two-dimensional dynamic spatial model is the best fitting model for Congresses 1 – 99.

What has happened in the past 20 years is that the second dimension of Congressional voting has slowly evaporated. As late as the 1990s the second dimension picked up differences within each of the parties over abortion, gun rights, and other social or lifestyle issues. For example, on the vexed issue of abortion each Party had a pro-choice and a pro-life faction. Hence, roll call votes on Abortion often cut through the parties along the second dimension. The same was true for gun control (see the spatial maps in this post). Hare and Poole show the second dimension disappearing in a variety of issue areas in this analysis.

The two figures below show that the extraordinary divisiveness that has marked American Politics since November 2000 has resulted in Congressional voting to collapse into a one dimensional near Parliamentary voting structure; that is, the parties are very unified as shown by Party Unity Scores. The first graph shows the correct classification for each House in 10, 2, and 1 dimensions using Optimal Classification. Note the dramatic convergence of all three measures during the past 20 years. This shows that almost every issue is voted along “liberal-conservative” (it is hard to make sense what this dimension means any more!) lines. Furthermore, no other period in American history shows this pattern.

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This graph shows the Aggregate Proportional Reduction in Error (APRE) ([{Minority Vote on a RC – Classification Error}/Minority Vote on a RC], summed over all the scaled roll calls). The APRE controls for the margins of the roll calls. The same pattern of collapse is shown here as well.

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Donald Trump will be the third consecutive President who is widely disliked by members of the opposite Party. Indeed, Trump’s personality coupled with the extraordinary party unity within each party will mean that American Politics will enter a phase that has never been seen before. We hope things do not melt down but we would not bet our mortgages on it!