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Evolution of political polarization in the US Congress

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Figure 1. Ridgeline plot of distribution of Liberal-Conservative scores by party in the US Congress, 1879-2023
This essay is on Wikiversity to encourage a wide discussion of the issues it raises moderated by the Wikimedia rules that invite contributors to “be bold but not reckless,” contributing revisions written from a neutral point of view, citing credible sources -- and raising other questions and concerns on the associated '“Discuss”' page. It links to an appendix containing a tutorial on how to create the plots discussed in this article. That tutorial makes updating these plots completely reproducible as long as the data source web site is not changed in a way that breaks the code. The tutorial is an R Markdown vignette compatible with the RStudio integrated development environment. Both RStudio and the R statistical software are free and open source with versions available for Microsoft Windows, macOS, and Linux.
Figure 2. Political polarization in the U.S. House, 1879-2023 from the 46th into the 117th Congresses
Figure 3. Political polarization in the U.S. Senate, 1879-2023
Figure 4. Political polarization in the U.S. House and Senate, 1879-2023
Figure 5. Political polarization in the U.S. House and Senate, 1879-2023

This article discusses the evolution of political polarization in the U.S. Congress in the post-Reconstruction era, 1879-2023 using the latest data from the Voteview website[1] and other sources. The appendix links to an R Markdown vignette, which should make it relatively easy for anyone to reproduce and update the figures in this article provided they have access to the Internet and a computer running versions of Windows, macOS, and Linux, supported by the R project for statistical computing.

Voteview has coded all roll call votes by members of the US House and Senate since the first Congress began in 1789, when George Washington was President. These roll call votes and other data have been combined and condensed into "Liberal-Conservative" scores for each member of Congress.[2]

Figure 1 extends a "ridgeline plot"[3] both backward and forward to 1879-2023 from 1963-2013. This plot shows the two parties becoming more polarized from 1879 to 1895 and 1905 then coming together during World Wars 1 and 2 and the Great Depression. The convergence ends in 1947, but they don't start diverging much until 1969. Since then, the Republicans have mostly become more Conservatives and the Democrats more Liberal, though over two thirds of the divergence has been due to Republicans becoming more Conservatives.

Figures 2 and 3 are updates with minor revisions of plots of mean difference between Republican and Democratic members of the US House and Senate created byPolitical Science professor Jeff Lewis at the University of California, Los Angeles.[4] Figure 4 superimposes Figures 2 and 3 on the same plot. Figure 5 extracts only the Republican and Democratic information from Figure 4, ignoring the difference between Northern and Southern Democrats.

Appendix. Companion R Markdown vignette

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Statistical details that make the research in article reproducible are provided in an R Markdown vignette on "Evolution of political polarization in the US Congress/plots".

This combines modifications of code from two sources:

  • Code dating from 2018-02-01 by Jeff Lewis,[4] available on GitHub.[5] This code inspired the readDW_NOMINATE function in the Ecdat package for R[6] and Figures 2-5 in this article.
  • Code used to produce a ridgeline plot.[3] revised to create Figure 1 above.

Notes

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  1. ↑ About the Voteview project, Wikidata Q130384333
  2. ↑ The "Liberal-Conservative" scores are called "nominate_dim1" in the data downloaded from the Voteview project website using the readDW_NOMINATE function in Yves Croissant; Spencer Graves (12 October 2022), Ecdat: Data Sets for Econometrics, Wikidata Q56452356, discussed further in the Appendix to this article. Voteview also allows users to view every congressional roll call vote in American history on a map of the United States and on a liberal-conservative ideological map including information about the ideological positions of voting Senators and Representatives. It was developed by Keith T. Poole and Howard Rosenthal, when they were at Carnegie Mellon University between 1989 and 1992 and is currently maintained in part by Political Science professor Jeff Lewis at the University of California, Los Angeles.
  3. ↑ 3.0 3.1 Michael Friendly; Howard Wainer (2021). A History of Data Visualization and Graphic Communication (in en). Harvard University Press. p. 250. Wikidata Q130475523. ISBN 978-0-674-97523-1. 
  4. ↑ 4.0 4.1 Jeffrey B. Lewis, Wikidata Q130384340
  5. ↑ Jeffrey B. Lewis, Polarization in Congress, Wikidata Q130384358.
  6. ↑ Yves Croissant; Spencer Graves (12 October 2022), Ecdat: Data Sets for Econometrics, Wikidata Q56452356
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