A long-term outlook based on conditions through mid-August (pre-Ryan/Akin/convention) shows a possible loss of control of the House by Republicans (Democratic takeover, 69% probability). The 2013 seat margin is headed to being narrower in either direction than the current Congress. An election based on today’s conditions would give Democrats 50-51 seats in the Senate and ~224 seats in the House — a knife’s edge in both chambers. This picture may change.
Caveat #1: The quality of House district data is unlikely to be as good as it was for the last two cycles because polling is down. But perhaps our data source (HuffPost/Pollster.com) or another aggregator will know better. This is a situation where a more complex approach for filling in missing data might help.
My first estimate will therefore be based on the national popular Congressional preference. A simple inspection of election returns for 2000-2010 shows the well-known fact that the D-minus-R House popular vote share is strongly related to the corresponding margin of House seats:
This graph shows that each 1.0% of popular-vote margin translates to a 6.0-seat advantage. Also, the intercept is nearly at the origin, i.e. no structural advantage for either side. A nearly-tied popular vote would translate to a nearly-tied House. So: 6.0 seats per percentage point.
However, note that individual data points deviate from the fit line by 7 to 17 seats in either direction. That is the first source of uncertainty, equivalent to 1-3 percentage points in national vote.
Next, let us use the generic Congressional ballot to estimate national popular vote. I will apply the Wisdom-of-Pollster-Crowds principle that has served so well in Presidential and Senate races, and assume that aggregating polls will approximate the actual popular preference.
Request to readers: Some have claimed that the generic-preference ballot does not predict seat outcomes. If you have recent quantitative data on this subject that is appropriate (i.e. that spans multiple pollsters, and surveys the last few Congressional elections), please add it in the comments section.
Here are this year’s polls. The average is a Democratic margin of 2.0%.
I have left out robopollers because Pollster.com uses averages, which does not allow me to easily use the median to address extreme pollster bias. The Internet polls mainly added noise, so they are left out too for clarity.
Update: a commenter took issue with the inclusion/exclusion of pollsters. My general approach, as longtime readers know, is to accept all polls. The above procedure was done strictly for display purposes, given Pollster.com’s software capabilities. Sadly, they do not use medians, which addresses the outlier problem. The median of all available generic Congressional preference polls since 8/15 is Democrats +2.0 +/- 2.0 % (median +/-SEM, n=7) (SEM corrected as per correspondence with Alan Cobo-Lewis, link to come).
Considering the possibility of movement in opinion between now and November and the fact that seat outcomes don’t precisely reflect popular vote, the total 1-sigma uncertainty is about +/-4%. This leads to a prediction of R+2% to D+6% (68% CI). This corresponds to anywhere from a 12-vote Republican majority to a 36-seat Democratic majority. The probability of retained Republican control is 31%, a knife edge situation.
All of the analysis above comes from data taken before the addition of Paul Ryan to the national ticket. Since then, the two most recent polls show the Democratic lead expanding to 4-8%. If that held, of course it would change the outlook. I’ll have more on this as it develops.