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Converting Gallup's Generic Ballot to Seats

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On Saturday, political scientists at the American Political Science Association conference in Washington, D.C. will debut their models for predicting results in the House of Representatives. In this article, I put forth my own model that translates Gallup's final generic ballot result into seats. While the generic ballot has generally not been fantastic at forecasting House outcomes, Gallup's final likely voter generic ballot poll has proven itself to be a great predictor in midterm elections.

In 2002, Alan Abramowitz created a model based off of Gallup's final likely voter poll in every midterm election since 1950. What I have done here is recreate that model and included 2002 and 2006 data. Abramowitz's model estimates the amount of seats the Republican party will gain by how many seats they won in the prior Congressional election, the party in the White House, and the Republican lead (or deficit) on the final generic ballot. I have also added my own variable: whether the party in the White House has been in power for more than one term. It is important to keep in mind that this model is based off only 14 elections and the final Gallup likely voter poll (before then Gallup's polls can be bouncy). If past trends hold, the model will do very well at predicting the 2010 final House seat count.

This simple model is
quite robust and explains a little over 97% of the variation
in the amount of seats won by Republicans in midterm elections from 1950-2006. In addition to gaining more seats when they do better on the generic ballot, Republicans are also more likely to perform well when Democrats control the White House, and the party in the White House has been there for more than a term. In 12 of the 14 elections*,
the regression's error is
5 seats or less. The model's error is never greater than 9 seats for any of the 14 elections.

So what type of lead do Republicans need on Gallup's final likely generic ballot to take back the House? Amazingly, they only need to be leading by 3% to be slated to garner 218 seats and win a majority by the slimmest of margins. If Republicans have a 6% lead, an error in estimate larger than this model has ever seen would be needed for Republicans not to gain back the House. A likely voter lead of 10% like Republicans had on Monday with registered voters, not likely voters who Republicans will do better among, translates into a cosmic 240 seats.

Other possible generic ballot margins to seat translations are

Screen shot 2010-09-02 at 4.13.25 AM.png

When Gallup makes the transition to a likely voter model, we will have a very good idea which party is destined to control the House chamber. Considering the current Republican position on the registered voter ballot, and how that has historically translated to the likely voter model, the Republicans look to be in mighty good position. Of course, November is still two months away.

*Gallup did not have a likely voter model in the fall of 1986.

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