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Bafumi, Erikson & Wlezien: Forecasting House Seats from Generic Congressional Polls

Topics: 2006 , The 2006 Race

(Editor's note: Today's Guest Pollster's Corner contribution comes from Professors Joseph Bafumi of Dartmouth College, Robert S. Erikson of Columbia University and Christopher Wlezien of Temple University. The post is based on a larger paper available for download here).

Although the Democrats hold a large advantage in generic ballot polls, there has been considerable uncertainty regarding whether the Democrats would win the most House Seats. Doubts are often expressed about the accuracy of the generic ballot polls. How district lines are drawn raises further doubts about whether the Democrats could win a sufficient majority of the vote to win the most seats. We estimate how the generic ballot "vote" translates into the actual national vote for Congress and ultimately into the partisan division of seats in the House of Representatives. Based on current generic ballot polls, we forecast an expected Democratic gain of 32 seats with Democratic control (a gain of 15 seats or more) a near certainty.

To begin with, we estimate a regression equation predicting the House vote in the 15 most recent midterm elections, 1946-2002, from the average generic poll result during the last 30 days of each campaign. The generic polls turn out to be very good predictors, as we have shown. Based on the current average of the generic polls (57.7% Democratic, 42.3% Republican) the forecast from this equation is a 55% to 45% Democratic advantage in the popular vote (1).

But would this mean that the Democrats also win the most seats? The Democrats winning 55% of the vote would represent a 6.4 percentage point swing from 2004, when they received 48.6%. If Democrats were to win exactly 6.4% more of the 2006 vote in every district than they won in 2004, they would win 228 seats. However, an average swing of 6.4% percentage points will be spread unevenly-sometimes more than 6.4% and sometimes less. Moreover, the prediction that the average vote swing will be 6.4% is itself subject to error.

We take these considerations into account by a set of simulations described in our larger paper. The simulations suggest that a predicted national vote surge of 6.4 percentage points would yield the Democrats 235 seats, for a 32-seat gain. This is 7 seats more than we would get with uniform swing.

A Democratic pickup of 32 seats might seem high to some readers. For a reality check, we compared our district level predictions from our simulations with the results of available district polls. The two sets of numbers match nicely. Our simulations might even underestimate Democratic strength in the sampled districts.

bafumi%20figure-sml.jpg

Of course if the generic ballot numbers shift as the election nears, the forecast should be revised according to the weight of new polling information. Figure 1 shows how the forecasts can shift with possible changes in the generic vote. If current trends in the Congressional generic ballot polling persist, the Democrats are near certain to win control of the House (2). But if the lead dips into the single digits, the Republicans can rekindle their hopes of holding on.

(1) As of October 24, PollingReport.com listed the results of 6 likely-voter generic ballot polls conducted during the final 30 days of the campaign, by CNN (2), ABC/Washington Post, Fox/Opinion Dynamics, Gallup/USA Today and Newsweek. The results for ABC/Washington Post listed on PollingReport.com actually are for registered voters, and we obtained the likely voter results from the news release posted on realclearpolitics.com. (Back to text).

(2) Readers conditioned to the idea that their districting advantage would allow the Republicans to govern with a minority of votes cast might be surprised that the threshold in terms of the national vote at which control is likely to revert to the Democrats is only 51%. The explanation is the partisan asymmetry in 2006 retirements. Among retirees who had faced major-party competition in 2004, 19 were Republicans and only 6 were Democrats. Strategic Republican retirements in anticipation of a Democratic wave would cause an electoral ripple even if the larger wave does not arrive. Our calculations are that if there is no vote swing whatsoever from 2004 to 2006, the Democrats would pick up 5 or more seats just from the greater number of Republican than Democratic retirements. (Back to text).

 

Comments
Alan Reifman:

Back in August, Pollster.com's own Charles Franklin posted an amazingly detailed analysis of the generic congressional ballot's (GCB) predictive validity from 1946-2004. He sliced and diced the GCB virtually every possible way, plotting the following relationships:

*The correlation (with year as each data point) between a party's showing in the final GCB and its share of the actual aggregate popular vote for the U.S. House.

*The correlation between GCB and number of seats won.

*The correlation between aggregate popular vote and number of seats.

There's an interesting twist near the end of the write-up, so be sure to read all the way to the end to see the "other shoe drop."

http://politicalarithmetik.blogspot.com/2006/08/votes-seats-and-generic-ballot.html

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