Ideological Cartography

As promised, here are my predictions for the 2010 Midterm Elections. The model parts with poll-based forecasting models in predicting that the Democrats will maintain control of the House. It predicts that Democrats will win between 217 and 238 seats, which translates into a loss of 19 to 40 seats. (The complete set of House predictions is available for downloaded as a .csv or a .xls.)

The histogram displays the predicted number of Democratic Seats from 10,000 bootstrap simulations.

If Congress operated more like a Westminster parliamentary system, fixating on which party will win a majority of seats would be more sensible. In such a setting, after assuming power the majority party (or coalition) is free to enact legislation with as little input from the minority as it pleases. This is not the case in Congress. The past two years have been a constant reminder to Democrats that even large electoral majorities do not grant similar levels of legislative control. Current theories of congressional behavior tell us that the position of the median member of Congress can be as important to policy outcomes as which party is in the majority. An advantage of my forecasting model is that it can predict ideological quantities of interest other than seat shares. For example, I can predict the position of the median member in the next Congress and the extent to which partisan polarization will increase or decrease.

The model projects that the position of the median House member in the 112^th Congress will be -0.05 with a 95 percent CI between -0.13 and .13. This represents a sizable shift to the right from the median legislator in the 111^th Congress, who was located at -0.24. Yet, this will only bring the median back to where it was during the 110^th Congress. To place this in perspective, the median House member in the 111^th Congress was in the region of Joe Baca (D-CA) and James Oberstar (D-MN). The model predictions place the median member for the 112^th Congress in the region of Arthur Davis (D-AL) and James Marshall (D-GA) but could be as far to the right as John McHugh (R-NY) or former Senator Lincoln Chaffee (R-RI), which is still very moderate. According to the model, even in the best-case scenarios, the House median will be much more moderate than what Republicans experienced during the 104^th-109^th Congress.

The figure below displays the trend lines for the median House member and the means for each party since 1990. Regardless of which party claims a majority after the election, the model projects an increase in partisan polarization. The mean Republican will experience its largest shift to the right ever recorded, while the mean Democrat also will move further to the left as Republican challengers pick off moderate Democratic incumbents. The general rule of thumb for this election is: the larger Republican gains, the greater the increase in polarization.

I report the model predictions faithfully here, but I remain somewhat skeptical of the model predictions for two reasons. The first is that the realm of campaign finance has undergone changes since the previous election cycle. Not accounting for independent expenditures by outside groups might have biased the model in favor of Democrats. On the other hand, the BCRA arguably represented a much larger shock to the campaign finance system than Citizens United, yet the model predictions for the 2004 Elections were right on target. Moreover, the model does not include any variables that relate to campaign expenditures; it only conditions on fundraising patterns, which remain largely unaffected by Citizens United. The second reason is that the model predicted Democrats would win about 10 seats fewer than they actually did in the 2006 Midterm Elections. It is difficult to determine whether this reflects the Mark Foley October surprise or a failure by the model to account for partisan momentum.

I suspect the model predictions are too generous to Democrats by about 8 to 12 seats. Even with the downward adjustment, I still predict the Democrats will retain their majority, but just barely. This is in line with Sandy Gordon’s forecasting model based on calibrated expert raters. Along with Sandy’s forecasts and the recent polls showing that oversampling of landlines can bias polls in favor of Republicans, my model provides additional evidence that poll-based forecasting models are overstating Republican gains. Fortunately, we won’t have to wait long to find out.