Ideology in the 113th Congress

The 113th Congress was sworn in this month, installing 47 new Democrats and 32 new Republicans.  Democrats gained eight seats in the House, but failed to capture the majority.  Because of newly drawn districts, Republicans hold 54% of the House despite having won only 48% of the vote.  In the Senate, Democrats gained two seats.  With 55 Senators, the Democrats still hold the majority, but again fall short of the 60 votes needed to surpass the ever present threat of filibuster.

So what has the election changed, if party control of Congress remains the same?  The ideological composition did.

To place this ideological shift in context, I’ve put together a graph that plots the ideological position of every member of Congress from the 73rd (elected in 1932) to the 113th (elected in 2012).  Democrats are shown in blue, and Republicans in red, arrayed from left to right based on their observed voting behavior.  (Click to enlarge.)


Looking at it this way, the recent polarization of Congress is starkly evident in the unprecedented, widening void between the parties.  Though it was once common for there to be some ideological overlap between the parties, that hasn’t been the case since 2004.  The polarization is deeper than just party affiliation, though.  The centrists of each chamber have been eroded over the course of the last decade.  In the election of 1994, the extreme edge of the House Republicans lurched to the right, eventually bringing along more and more of its members in both chambers.

The other element of the graph is a metric of my own calculation.  The shaded boxes indicate the narrowest majority coalition possible in each session, ideologically.  Every box represents a majority of members spread across a particular ideological range.  In the last four sessions of the Senate, in recognition of the increased use of the filibuster, I’ve also shown the narrowest sixty vote coalition. Conceptually, it is more difficult to pass a bill with a wider coalition, because you need to get more ideologically disparate people to agree.

Considering the coalitions side-by-side, you can see how difficult it must be to craft a bill that can at the same time satisfy a majority of the House, a super majority of the Senate, and the White House.  Looking back on the last century, no other Congress was as divided as the 112th.  Unless the Senate reforms the filibuster and the House can routinely hold votes that cross the partisan gulf like the fiscal cliff vote did, the 113th Congress looks to be just as divided.


This graph is based on the DW-NOMINATE Common Space data, which has meticulously analyzed the votes cast by each member of Congress, all the way back to the first.  Because so many members serve for multiple terms, and sometimes in both chambers, it’s possible to compare the ideological composition of each Congress, arraying every member on a left-right axis.  (The data set also includes a second dimension, corresponding roughly to North-South, which only seemed to be important during the Civil War and during the struggle for Civil Rights.  However, there have been some interesting/frightening indications that this second axis is making a comeback.)

The calculation of the narrowest coalitions depends only on the first dimension of ideology.  Because of the overlapping ideology of the parties in years past, and the onetime importance of the second dimension, the coalitions I have highlighted are not always found solely among the majority party.  Also, to accommodate the intra-session changes in membership when calculating coalitions (e.g., due to resignations or death), I weighted each member (and the Vice President) by number of votes cast.

For the 113th Congress, direct observation of ideology is impossible, because they haven’t been casting votes.  In many cases, however, we do have past observations: members were not up for reelection (in the Senate), or were reelected, or had served in prior years, or had served in the other chamber.  For the newly elected members, it is possible to forecast their ideology based on a regression of past voting behavior by party, sex, self-identification as a member of the Tea Party, representative from a state of the South, and the share of votes for the President in each district or state.  Here are the regression results, from the team that maintains the DW-NOMINATE data set, for the House and the Senate.

The Effects of the 2012 Election

Data Display

I have previously commented on another attempt to display this data by the XKCD web comic.  Although I love XKCD’s penchant for data display, I had some problems with his design choices in this case.  I chose to represent ideology, which is a continuous variable, on a continuous scale from left to right, unlike XKCD’s categorical approach with six shades of color.  The XKCD approach more clearly showed the number of members from each party by stacking them left and right.  I tried to represent the same information less precisely by overlaying transparent markers that indicate the density of each caucus, and also by highlighting the members that constitute a majority.  I hope that you find this approach a little easier to digest.

7 thoughts on “Ideology in the 113th Congress

  1. Trevor – interesting graph. I like your approach much better than the XKCD approach. One other thing – who’s the Democrat in 1974-1982 that’s much more conservative than the majority of the Republican delegation?

    • That was Larry McDonald of Georgia, who was the second president of the John Birch Society, an admirer of McCarthyism, and who thought Martin Luther King was associated with secret communist agents. According to Wikipedia, the Democratic party in his district once censured him “for the dishonorable and despicable act of calling himself a Democrat.” Just goes to show you that party affiliation masks all sorts of ideological diversity…

      I’m glad you like the graph. If I had the ability, I’d make it interactive, so you could hover over each data point and see who it was. In the meantime, I’m happy to talk about the outliers!

  2. Really interesting stuff & great graphs that really illustrate the type of polarization you referenced. It’s unfortunate that gerrymandering has had a hand in the disparity between percentage of votes received and actual representation, regardless of which party benefits. I believe my own home district back in PA stretches across five counties!

  3. Fascinating read! I’m particularly interested in the comeback of the “second dimension.” IMO, the GOP’s recent lurch to the right meshes neatly with the Civil War era politics of the Confederacy.

  4. Pingback: Ideology in the 113th Congress | The Secular Jurist

  5. The chasm between Rs and Ds in the house clearly began in the 90s and became particularly pronounced in the 94 election as you write. But it’s also noteworthy that the narrowest majority coalition–which you outline in grey– last crossed party lines more a decade earlier, indicating that the polarization began much earlier. It may just take some time for the party outliers to coalesce or leave office.

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