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Using the Generic Vote to Forecast U.S. House and Senate Elections

By Alan Abramowitz

With the 2006 midterm election fast approaching, Democrats’ chances of regaining control of the House of Representatives remain unclear. On the one hand, national political conditions appear to be more favorable for Democrats than at any time since the Republican takeover of the House in 1994. A pickup of only 15 seats would give Democrats control of the House in 2007 and, since the end of World War II the average midterm seat loss for the president’s party is 24 seats. Moreover, when the president’s approval rating is below 50 percent, the average midterm seat loss is 38 seats and according to data compiled by pollingreport.com, George Bush’s average approval rating during the month of April was only 36 percent.

Recent national polls also show Democrats with a strong lead in the “generic vote” for Congress. Between September, 2005 and April, 2006 there were 48 national polls asking Americans which party they preferred in the 2006 House elections. Democrats led in every one of these polls with an average advantage of about 10 percentage points among registered voters. In seven polls during April, the average Democratic advantage was close to 12 percentage points. This is the largest margin Democrats have enjoyed in the generic vote since the early 1990s, before the Republican takeover of the House.

So if the national outlook for the Democrats is so rosy, why are many pundits and journalists skeptical about the Democrats’ chances? The answer is that a midterm election is not just a national election. It is also a collection of 435 individual House races and 33 individual Senate races and right now the evidence from those individual races does not clearly point to big Democratic gains in November....

In order to predict the outcome of the 2006 House elections, I create a forecasting model incorporating both national political conditions and the actions of strategic politicians. Pre-election Gallup Poll data on the generic vote and presidential approval are used to measure national political conditions and data on open seats and challenger quality are used to measure the actions of strategic politicians. The model is tested with data on U.S. House elections between 1946 and 2004. A simpler model based only on national political conditions is tested with data on U.S. Senate elections from the same period.

The dependent variable in the House forecasting model is the change in the percentage of Republican seats in the House of Representatives. The model includes seven independent variables. The percentage of Republican seats in the previous Congress is included to measure the level of exposure of Republicans compared with Democrats in each election—the larger the percentage of Republican seats in the previous Congress, the greater the potential for Republican losses. Dummy variables for Democratic and Republican midterm elections are included to capture the effect of anti-presidential-party voting in midterm elections. Net presidential approval (approval – disapproval) in early September is included to measure public satisfaction with the performance of the incumbent president, and the difference between the Republican and Democratic percentage of the generic vote in early September is included to measure the overall national political climate. The actions of strategic politicians are measured by two variables: the difference between the percentages of Republican and Democratic open seats and the difference between the percentages Democratic and Republican quality challengers (defined in terms of elected office-holding experience)....

....Based on a net approval rating for President Bush of -20, a Democratic advantage of 10 points in the generic vote, and a Democratic advantage of 2 percent in open seats, the model predicts a Democratic gain of 20 seats in the House of Representatives with no Democratic advantage in challenger quality. A modest 3 point Democratic advantage in challenger quality, which is consistent with the district-level analysis presented in the April 28, 2006 Cook Political Report, would increase the predicted Democratic gain to 27 seats.

Read the full working paper by Alan Abramowitz here.