The Incredible Hulk and the 2010 election

Last week in a post here, I quoted Jay Cost of the Weekly Standard as follows:

Allocating the undecided voters proportionally, Bruce Banner gets a two-party vote of 54.5 to 45.5.  That’s a nine-point GOP win, in line with a prediction of a historically high Republican caucus, say 240 seats (which is what I actually did predict last week).

Incredible Hulk.  The Hulk has problems with this analysis.  It tosses out what has historically been the best estimator of midterm congressional results, the Gallup generic ballot likely model.  This year Gallup is calling it the “traditional” model, but in every midterm before this, it was the only likely voter model.

Only once in 60 years has the Gallup generic ballot underestimated Democratic strength by a significant amount – by 2% in 2006.  On average, it slightly overestimates the Democrats, by 0.7%.

Here is what he is seeing this morning based upon Gallup showing a 15 point GOP likely voter advantage:

My internal conflict between “Bruce Banner,” who predicts a 1994-style scenario, and “The Incredible Hulk,” who thinks 2010 will be as Republican as anything since the 1920s, has been resolved.

Hulk wins. Here’s why.

The generic ballot has become very important this cycle, with more pollsters than ever offering up their version of the question, which essentially asks if you plan to support a generic Republican or Democrat for Congress.

A common inferential error with this metric comes in reading it like presidential polls. The latter tend to be very accurate, but the generic ballot polls have exhibited a systematic tilt or skew to the Democrats, at least in midterm elections.

This is most notably true for generic ballot polls that query registered voters, as opposed to likely voters. You can see this in particular with Gallup’s track of registered voters, here at the bottom of this page.

But the problem goes beyond that.  In fact, the final pre-election generic ballot numbers of likely voters tend to exhibit a Democratic skew as well.
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What we see here is an average skew to the Democrats of about three points. That holds true for good Republican years (1994 and 2002), good Democratic years (2006), and split decision years (1998).

In fact, there has really only been one pollster in 15 years that has not exhibited a systematic partiality, and that is the Gallup poll. This was my point last week, that the Gallup final likely voter generic ballot number is extremely accurate. And as you can see it is more accurate, on average, than any other poll. In fact, it’s even substantially more accurate than an average of all the other final likely voter polls. In three out of four years, tossing out all the other polls and following only Gallup would have gotten you closer to the actual result.

This suggests, in turn, that our best approach to minimizing error when it comes to predicting the final popular vote spread is to favor heavily Gallup’s final likely vote projection.

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A victory of 15 points suggests Republican gains well in excess of my previous estimate of 61 seats. The Abramowitz model suggests a pickup of about 76 seats, but I wouldn’t take that at face value. After all, there is a great deal of uncertainty because we are dealing with unprecedented results, which Gallup is quick to acknowledge. A Republican vote margin of 15 points would more than double the party’s 1994 victory and it is about double its 1946 victory. In fact, you have to go all the way back to 1928 to find an election where the popular vote margin resembled anything close to what Gallup is predicting.

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Go here to read the entire article.  When it comes to just how many seats the GOP will gain tomorrow, we have clearly entered Here Be Dragons territory based upon the Gallup results.