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The Polarization of the Congressional Parties

Updated 30 January 2016

Recent Papers

Adam Bonica, Nolan McCarty, Keith T. Poole, and Howard Rosenthal. 2015. "Congressional Polarization and its Connection to Income Inequality." American Gridlock: The Sources, Character, and Impact of Congressional Polarization, Chapter 16, pp. 357-377, edited by James A. Thurber and Antoine Yoshinaka. New York: Cambridge University Press, 2015.

Christopher Hare and Keith T. Poole. 2014. "The Polarization of Contemporary American Politics." Polity, 46:411-429.

Adam Bonica, Nolan McCarty, Keith T. Poole, and Howard Rosenthal. 2013. "Why Hasn't Democracy Slowed Rising Inequality?" Journal of Economic Perspectives, 27(3): 103-24.

"Picture of a Polarized Congress" (UGAresearch, the research magazine of the University of Georgia, 2012) (PDF: Picture of a Polarized Congress)

Nolan McCarty, Keith T. Poole, Thomas Romer, and Howard Rosenthal. 2010. "Political Fortunes: On Finance and its Regulation." Daedalus, 139-4:1-13. (This paper is the protoplasm of our 2013 book: Political Bubbles: Financial Crises and the Failure of American Democracy)

Nolan McCarty, Keith T. Poole, and Howard Rosenthal. 2009. "Does Gerrymandering Cause Polarization?" American Journal of Political Science, 53 (July):666-680.

"Growing Apart: The Mathematical Evidence for Congress' Growing Polarization" (by Jordan Ellenberg, Slate Magazine, 26 December 2001)

Keith T. Poole and Howard Rosenthal. 1984. "The Polarization of American Politics." Journal of Politics, Vol. 46, No. 4 (November).

Graphs

Below are graphs of the difference between the Republican and Democratic Party means on the first DW-NOMINATE dimension from the end of Reconstruction through the the first session (2015) of the 114^th Congress. This difference in first dimension means is a good measure of the level of political polarization. By this measure polarization is now at a post-Reconstruction high in the House and Senate.

With few exceptions, roll call voting throughout American history has been simply structured. Only two dimensions are required to account for the great bulk of roll call voting. The primary dimension is the basic issue of the role of the government in the economy, in modern terms liberal-moderate-conservative. The second dimension picked up regional differences within the United States -- first slavery, then bimetalism, and after 1937, Civil Rights for African-Americans. With the passage of the 1964 Civil Rights Act, the 1965 Voting Rights Act, and the 1968 Open Housing Act, this second dimension slowly declined in importance and is now almost totally absent. Race related issues - affirmative action, welfare, Medicaid, subsidized housing, etc. - are now questions of redistribution. Voting on race related issues now largely takes place along the liberal-conservative dimension and the old split in the Democratic Party between North and South has largely disappeared. Voting in Congress is now almost purely one-dimensional - a single dimension accounts for about 93 percent of roll call voting choices in the 114^th House and Senate - and the two parties are increasingly polarized.

Polarization declined in both chambers from roughly the beginning of the 20th Century until World War II. It was then fairly stable until the late 1970s and has been increasing steadily over the past 25 years. Our (Poole and Rosenthal, 1997) original D-NOMINATE estimation ended with the 99th Congress. Interestingly, Congresses 100- 114, if anything, mark an acceleration of the trend (especially in the House). Note, however, that the acceleration is smooth and does not show a particular jump in polarization induced by the large Republican freshman class elected in 1994. Polarization in the House and Senate is now at the highest level since the end of Reconstruction.

In addition, the percentage of moderate Representatives and Senators continues to plummet. In the House the percentage of moderates (-.25 to +.25 on the first DW-NOMINATE dimension) has declined to about 10 Percent in both Chambers.

Below are a number of graphs that display the polarization of the parties in Congress since 1879. After the graphs we have links to the data used to construct them in a variety of formats.

The format of the Polarization Data files is:

 1.  Congress Number
 2.  First Year of the Congress
 3.  Difference in Party Means - first dimension
 4.  Proportion Moderates
 5.  Proportion of moderate Democrats (-0.25 to +0.25)
 6.  Proportion of moderate Republicans (-0.25 to +0.25)
 7.  Overlap
 8.  Chamber Mean - first dimension
 9.  Chamber Mean - second dimension
10.  Democratic Party Mean - first dimension
11.  Democratic Party Mean - second dimension
12.  Republican Party Mean - first dimension
13.  Republican Party Mean - second dimension
14.  Northern Republican Mean - first dimension
15.  Northern Republican Mean - second dimension
16.  Southern Republican Mean - first dimension
17.  Southern Republican Mean - second dimension
18.  Northern Democrat Mean - first dimension
19.  Northern Democrat Mean - second dimension
20.  Southern Democrat Mean - first dimension
21.  Southern Democrat Mean - second dimension