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In the home stretch, I wrote that midterm polling is far less accurate than in Presidential years. Today, in The American Prospect, I detail how this year’s polling errors are correlated with voter turnout, which was the lowest since 1942, as based on Michael McDonald’s tabulation so far. In 2014, Democrats underperformed expectations by over 5 percentage points on average, the largest such error in over 20 years. State by state, underperformance was correlated with low turnout. This suggests that voter apathy on the Democratic side was a significant factor in the 2014 election. Here’s the article.

From a polling standpoint, estimating turnout is likely to be a major source of systematic error. Here are some details of this year’s GOP “bonus.”

In the Senate, the GOP overperformed by 5 percentage points

Republican performance above and beyond pre-election Senate polls can be estimated in two ways.



The first is pretty simple: just calculate the difference between outcomes and polls for states that were in contention. This gives a “bonus” for Republicans of 5.2 ± 1.1% (mean ± SEM; SD=4.2%). Comparing with past Senate polling errors, this is the largest error in the FiveThirtyEight database going back to 1990. The next-largest error was a 4.9% bonus for Democrats in 1998.

The second way is to use the Meta-Margin, the amount of swing needed to create a tie for control. This can be calculated by arranging the races in order of winning margin in order, and calculating what the “tipping-point” margin is between the 50th and 51st GOP win. That’s a little ambiguous because the Louisiana runoff has not occurred yet. But if we assume that the tipping-point margin is halfway between Georgia and Louisiana, it would be R+5.8%. Compared with the pre-election Meta-Margin of R+1.0%, that’s a Republican bonus of 4.8%.

I note that one state where turnout cannot account for the polling error is Iowa. There, turnout was quite robust – and Joni Ernst (R) outperformed by about six percentage points. This is worth examining. Nate Cohn at the NYT suggests it’s a sign that Iowa is newly competitive. That is certainly possible. One possible reason is early voting in that state, an area where Republicans did much better than in 2010.

Similar overperformance in the House

In pre-election polls, the generic Congressional ballot showed a Republican lead of 1.5 percentage points. The House vote isn’t done being counted yet, but David Wasserman and his team at the Cook Political Report currently have Republicans leading the national popular vote by 6.8%. (They also confirm that turnout was exceptionally low, down by about 42% from the 2012 election.) The difference between polls and the actual House vote is a bonus for Republicans of 5.3%, similar to the Senate estimates.

Republicans gained at least 14 seats, with 6 more races outstanding (in these six, 3 Democrats lead and 3 Republicans lead). A gain of 17 seats is larger than expected (0 to 12 seats gained). However, it is smaller than 26 seats, which is the median post-World War II midterm gain by the President’s opposition party.

Finally, as I wrote last Wednesday, gubernatorial races showed a smaller bonus for Republicans, about 2 percentage points. I don’t know yet why this is smaller. One possibility is that voters are more likely to cross party lines when voting for governor. Many states have a history of electing governors from the non-dominant party. Another possibility is that when pollsters are focused on a particular state’s race, they are better at gauging likely voters than when they have to build a model that applies to many states at once.

It occurred to me to ask whether the direction of the correlation between turnout and polling bias was always the same. The answer to that is no. In 2006 and 2010, Democrats did better than expected in low-turnout states. The absolute magnitude of pollster error was similar – but the direction was different. In those years, it was Republicans who tended to show up less often than expected. I’ll post a graph of that later.

Currently, I think the general principle is that pollsters have a tough time identifying people who are marginally committed to voting. In 2010, Republicans seemed angry with the President and Congressional Democrats – but evidently not as mad as pollsters thought.

One interesting note: as David Wasserman points out, Republicans won the popular vote in 2010 by 6.6%, nearly the same margin as in 2014. Yet Republicans led the generic Congressional ballot in 2010 by over 10 percentage points. This year is a particularly vivid example of the difference that voter turnout can make.

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P.S. Later I will do postmortem examination of long-term forecasting. It’s not far from my mind. More later.