Now that the 114th House has conducted 66 roll call votes and the 114th Senate has voted 53 times, we now have enough data to take a very preliminary look at ideological polarization in the new Congress. Following standard practice, we use first dimensionDW-NOMINATE scores as measures of legislators’ liberal-conservative positions.

The first two plots below show the mean score of Democrats (Northern and Southern) and Republicans in both chambers over time. Though we are hesitant to put too much stock in these results at this early date, it looks like polarization (the ideological distance between Democrats and Republicans) is on pace to take a slight uptick in the 114th House but may level off in the 114th Senate. In the Senate, this is due to a slight shift in the Republican mean back to the center. Some of this may be due to the exit of Senator Tom Coburn (R-OK) [whose score is 0.792] and the entry of Senators Cory Gardner (R-CO) [0.216] and Mike Rounds (R-SD) [0.398]. There may also be a procedural aspect as Majority Leader Mitch McConnell (R-KY) has allowed several votes on amendments.

Interestingly, the defeat of Democratic moderates like Senators Mark Begich (D-AK), Kay Hagan (D-NC), Mary Landrieu (D-LA), and Mark Pryor (D-AR) has had very little effect thus far in moving the Democratic mean leftward. To some degree, their exit may be counterbalanced by the retirements of liberal Democrats like Senators Tom Harkin (D-IA) and Carl Levin (D-MI).

The second set of images shows the percentage of Democratic and Republican legislators in both chambers with DW-NOMINATE scores less than -0.5 or greater than 0.5, making them more ideologically extreme. There has been little change in these values between the 113th and 114th Congresses, with the exception of a marked increase in the proportion of House Republicans with scores greater than 0.5.

These image are from a new stand-alone DW-NOMINATE that can be run daily as new roll calls are cast. We will have more to say about this software at a later date.

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Below we use updated DW-NOMINATE scores to plot the House’s266-153 vote and the Senate’s62-36 to approve theKeystone XL oil pipeline.

28 House Democrats and 9 Senate Democrats joined all voting Republicans in supporting the measure. As seen in the plots below, NOMINATE does a good job of modeling voting patterns on the basis of ideology, with moderate Democrats in both chambers predicted to defect from the rest of their party.

These image are from a new Stand-alone DW-NOMINATE that can be run daily as new roll calls are cast. We will have more to say about this software at a later date.

Click images to enlarge

Updated 30 January 2015

Below is the vote to elect John Boehner (R-OH) as Speaker of the House. The actual roll call was 216 for Boehner, 164 for Nancy Pelosi (D-CA), 24 Republicans voted for other Republicans, 4 Democrats voted for other Democrats, Babin (R-TX) voted Present, and 25 members did not vote. Grimm (R-NY) resigned before the vote was taken.

The 24 Republicans who did not vote for Boehner were treated as voting for Pelosi and the 4 Democrats who did not vote for Pelosi were treated as voting for Boehner. This was done solely for display purposes. Most of the non-voters were Democrats — 20 of the 25. The opposition to Boehner was from the right-wing of the Republican caucus and the 4 Democrats opposed to Pelosi were relatively moderate.

This image is from a new Stand-alone DW-NOMINATE that can be run daily as new roll calls are cast. We will have more to say about this software at a later date.

Click image to enlarge

Below we plot the first dimensionDW-NOMINATE Common Space scores of the presidents in the post-war period, which we refer to as the “presidential square wave” due to its shape. An ideological score is estimated for each president throughout the entirety of their tenure in office by scaling their “votes” on a subset of roll call on which they announce a position (measured usingCQ Presidential Support Votes). Negative DW-NOMINATE scores indicate greater liberalism and positive scores indicate greater conservatism. The presidential scores are directly comparable across time and with members of Congress.

These presidential DW-NOMINATE scores are estimated using all available CQ presidential support roll calls through 2013. CQ does not issue all of its presidential support roll calls until the print version of its congressional roll call guide comes out, and so only a fraction of the 2014 votes are available.

Very little has changed from thelast presidential square wave. President Obama fits the spatial model estimated by DW-NOMINATE extremely well, with over 95% of his “votes” correctly classified. Obama has moved very slightly leftward (-0.367) and is now just to the left of LBJ (-0.346) and right of Truman (-0.368), though this trio is virtually ideologically indistinguishable. President Eisenhower is the most moderate president (0.293) of the post-war era.

Among members of the 113th Congress, President Obama is very ideologically close to Representatives Stephen Lynch (D-MA) [-0.364] and Loretta Sanchez (D-CA) [-0.369] in the House, and Senators Patty Murray (D-WA) [-0.367] and Mark Udall (D-CO) [-0.359] in the Senate.

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Below we show the polarization of the Political Parties for the 1879 through 2014 period (46th to 113th Congresses). Polarization is measured by the distance between the means of the Democrat and Republican Parties on the first (Liberal vs. Conservative)DW-NOMINATE dimension. Polarization is now at a Post-Reconstruction high in both the House and Senate.

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Below are the Party means for the House on the firstDW-NOMINATE dimension. For the Democrats we show the means for the Northern and Southern wings of the Party (We use the CQ definition of South; the 11 States of the Confederacy plus Kentucky and Oklahoma). In the past three Congresses the difference between the Northern and Southern Democrats has disappeared.

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Below are the corresponding Party means for the Senate on the firstDW-NOMINATE dimension. The pattern is essentially the same as the House. However, the Southern Democratic Senators as a group tend to be more moderate than their Northern counterparts.

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Below are the Party mean graphs for the House and second for the secondDW-NOMINATE dimension. This dimension usually picks up regional differences between the two major parties. Before the Civil War the second dimension picked up the North vs. South division on Slavery. In the Post Reconstruction period the second dimension picked up regional differences on soft vs. hard Money (bimetalism, gold and silver) and beginning in the late 1930s Civil Rights. In the past 20 years the second dimension has faded in importance and issues that used to divide the parties internally — e.g., gun control, abortion — now load almost entirely on the first dimension. This is explored in detail inKeith T. Poole and Howard Rosenthal (2007)Ideology and Congress. Note that in the figures below the parties have almost converged on the secondDW-NOMINATE dimension. Voting in Congress is almost entirely one-dimensional. The first dimension now accounts for over 93-94 percent of the roll call votes.

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Below we useOptimal Classification (OC) in R to plot the House’s219-206 vote on a $1.1 trillion spending package (the so-calledCromnibus). The vote split both party caucuses, with House Republicans voting 162-67 in favor of the bill and House Democrats splitting 57-139 against it.

The bill angered ideologues on both sides, and the result was a “two-ends-against-the-middle” vote in which a coalition of more moderate Republican and Democratic legislators united to pass the measure. The mean first dimension (representing liberal-conservative position) score of Democrats who voted Yea is -0.41, compared to a more liberal mean score of -0.49 for Nay Democrats (p < 0.01). The mean first dimension score of Yea Republicans is 0.38, compared to a more conservative mean score of 0.44 for Nay Republicans (p < 0.01).

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Note: The plot show only 217 Yea and 203 Nay votes because several newly-elected members have not yet cast enough votes to be included in the scaling.

Below we useOptimal Classification (OC) in R to plot the House’s252-161 vote and the Senate’s59-41 vote on approving the Keystone XL pipeline. The Senate fell one vote short of approving the measure, which was designed in partto help Senator Mary Landrieu’s (D-LA) prospects in her upcoming runoff against Rep. Bill Cassidy (R-LA).

31 House Democrats and 14 Senate Democrats joined with all voting Republicans in both chambers in support of the measure. The first dimension (representing legislators’ liberal-conservative positions) does a good job at capturing this internal divide among House and (especially) Senate Democrats. Three of the Democratic Senators who voted Yea were defeated in their re-election races earlier this month — Senators Mark Begich (D-AK), Kay Hagan (D-NC), and Mark Pryor (D-AR) — and these are among the more moderate members of the Senate Democratic Caucus. This helps to illustrate why polarizationis likely to increase in the next (114th) Senate.

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Thoughpartisan polarization has increased dramatically in both chambers of Congress over recent decades, it has beenmore pronounced in the House than the Senate. There is reason to believe that yesterday’s midterm elections will narrow this gap, increasing the ideological distance between Democrats and Republicans in the 114th Senate.

Below we useDW-NOMINATE Common Space scores to plot the ideological positions of members of current, 113th Congress. We use Common Space scores because they allow for direct comparability between members of the House and Senate. The ideological distributions of Democrats and Republicans in the House are shown with the light blue and red lines, while Senate Democrats and Republicans are plotted using the dark blue and red lines.

We highlight the ideological locations of the nine Senate Democrats whose seats flipped (or are likely to flip) to Republicans: Senators Mark Begich (D-AK), Mark Pryor (D-AR), Mark Udall (D-CO), Tom Harkin (D-IA), Mary Landrieu (D-LA), Max Baucus (D-MT), Kay Hagan (D-NC), Tim Johnson (D-SD), and Jay Rockefeller (D-WV). We can also plot the positions of five of the Republican Senators-elect (or, in Louisiana, the front-runner for the December 6 runoff) who have served in the House and have comparable Common Space scores: Reps. Tom Cotton (R-AR), Cory Gardner (R-CO), Bill Cassidy (R-LA), Steve Daines (R-MT), and Shelley Moore Capito (R-WV).

The 2010 midterms devastated the ranks of moderate Blue Dogs in the House. To a lesser degree, the 2014 midterms will have a similar effect on the Senate Democratic Caucus. Six of the Democratic Senators shown are among the seventeen most moderate Democrats in the current, 113th Senate (using the Common Space scores). With the exception of Senator-elect Shelley Moore Capito (R-WV), their replacements do not appear to be symmetrically moderate. The other four Republican Senators-elect for whom we do not have ideological scores (Dan Sullivan in Alaska, Joni Ernst in Iowa, Thom Tillis in North Carolina, and Mike Rounds in South Dakota, seem likely to be at least as conservative as the median member of their party’s caucus). Indeed, the freshman Republicans in the 114th Senate appear primed to move their party’s distribution closer to that of House Republicans.

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Below we useOptimal Classification (OC) in R to plot the House’s273-156vote and the Senate’s78-22vote to approvePresident Obama’s request to arm and train Syrian rebels. As seen below, both votes split the parties, with 85 Democrats/71 Republicans in the House and 10 Democrats/12 Republicans in the Senate voting Nay.

There are not clear liberal-conservative divides within the parties in the House, but the second dimension (on the vertical axis) appears to do a better job at classifying Yea and Nay votes. Although the substantive meaning of this dimension is not entirely clear in the contemporary Congress, we havepreviously suggested that it may represent an insider vs. outsider divide. On the Senate vote, however, there are clearer ideological splits (especially in the Republican Party), with more conservative Republicans and more liberal Democrats being more likely to oppose the resolution. We saw a similar pattern inSenators’ announced positions on Syria last year.

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There is little question that the parties in Congress have moved ideologically further apart over recent decades. But has polarization been symmetric (with both parties moving away from the center at roughly equal rates), or has it been assymetric (driven more by Republican than Democratic movement)? DW-NOMINATE scoresunambigously support the claim of asymmetric polarization, but somealternate ideological measures suggest that polarization has been more symmetric.

Below we useCommon Space DW-NOMINATE scores to demonstrate that the finding of asymmetric polarization is not an artifact of the time trend allowed in regular DW-NOMINATE scores. With Common Space scores, a single score is estimated for each member based on their entire (often decades-long) congressional roll call voting record. Separate scores are not estimated for each member in each Congress. Hence, the only factor that could be driving polarization in the graphs below is member replacement (for instance, when a more conservative/liberal Senator replaces a more moderate Republican/Democrat).

The 10%/90% ranges refer to the 10th and 90th ideological percentiles of Democrats and Republicans in both chambers. That is, 80% of Democrats and Republicans in the corresponding chamber fall within these bands. We see the same polarizing trends as when using standard DW-NOMINATE scores: the House has polarized more than the Senate, and Republicans have moved further away from the center than Democrats. The final graph below shows a histogram of the House and Senate distributions in the current 113th Congress, with the positions of some key past and present figures denoted. There is a clear hump on the right that is comprised of mostly new members of Congress like Senators Ted Cruz (R-TX) and Rand Paul (R-KY).

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