In TalkingPointsMemo is a rundown of political-science models of this year’s Congressional campaign. Such models are research tools that use pre-campaign fundamentals to test a hypothesis about how a campaign “ought” to turn out.
Today I point out that the Highton/Sides/McGhee Senate forecast has, in some sense, already been confirmed: it is essentially identical to PEC’s long-term forecast, which was based on polls from June to now. In that sense, the TPM article didn’t mention a pretty interesting fact: the match between our polls-only analysis and at least one non-polls-based model.
Highton et al. say their “current estimate is for the Republicans and Democrats to each control 50 seats with a 95% prediction interval of 48 to 52 seats.” Above, shown in yellow and red, are the PEC forecast as of September 9th; note specifically the prediction of 50.3 ± 0.7 Democratic+Independent seats. My “±0.7″ corresponds to the red “1-sigma interval,” and the yellow zone is approximately the 95% prediction interval: 50.3 ± 1.4 seats – or 48.5 to 52.1 seats.
A widely held view in political science is that campaigns play the role of driving public opinion toward some natural endpoint. This nearly perfect match between model and polling reality since summer suggests that in 2014, the campaign started reflecting fundamentals months ago. Of course, we’re at a point in the campaign when polls provide a much sharper view than pre-campaign fundamentals. Today, the PEC November prediction is for Democrats and Independents to have 50 or more seats with 63% probability. If outcomes match polls, the most likely outcome is looking like a 50-50 split with Republicans.
One technical note: the upheaval in Kansas in early September drove the PEC prediction upward slightly. Not counting the “Orman offset,” PEC’s long-term prediction was actually 48.0-51.6 seats. That’s still entirely consistent with the Highton et al. model.