Jeff Lewis and Keith Poole, 24 August 2015

Below we show the Senate Chamber and Party Medians on the first DW-NOMINATE dimension for the first 113 Congresses with 95 percent credible intervals based upon 250 parametric bootstrap trials. (The Working Paper was later published in Political Analysis, 17(3):261-275, 2009).

The graph below shows the Senate Median from 1789 to 2014 (Senates 1 – 113). During the three stable two-party periods in American history the Senate Median will be in the majority party. For example, in the very early period when the Federalists dominated the Jeffersonians, the Senate Median was to the right. The changes in the Senate Median over time follow the historical analysis of Poole and Rosenthal in Congress: A Political-Economic History of Roll Call Voting, chapters 4 and 5.

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The graph below shows the Party Medians and 95 percent credible intervals for the Federalist-Jeffersonian Republican Party System (1789 – 1811). The movement to the right by the Federalists should not be over interpreted since they only had seven seats in the 11th (1811-12) Senate. The opposition of the Federalists to the War of 1812 resulted in the collapse of the Party during The Era of Good Feelings.

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The graph below shows the Party Medians and 95 percent credible intervals for the Whig-Democrat Party System (1827 – 1848). The Democrats were the dominate party through this period but the second dimension split the two parties along North vs. South lines. The conflict over Slavery and its extension to the territories caused the collapse of the Whig Party after the Compromise of 1850 and despite the efforts of Senator Stephen Douglas (D-IL) the admission of more states during the 1850s did not settle the North-South divide within the Democratic Party.

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Finally, the graph below shows the Party Medians and 95 percent credible intervals for the Republican-Democrat Post-Reconstruction Party System (1879 – 2014). The trend to greater polarization in the modern era is clearly evident at the end of the series.

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