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


Alan I. Abramowitz
Department of Political Science
Emory University
Atlanta, Georgia 30322



Abstract
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.  



	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.

	Because of the tremendous advantages enjoyed by incumbents in House elections, 
open seats generally present the best opportunities for opposition party takeovers.  But so 
far only 18 Republican incumbents, along with nine Democratic incumbents, are not 
seeking reelection in 2006.  And many of those retiring Republicans represent safe 
Republican districts.
  
	Only 12 House Republicans won by a margin of less than 10 percentage points in 
2004 and only 16 House Republicans represent districts that were carried by John Kerry.  
So among Republican incumbents, there is relatively little low-hanging fruit for 
Democrats to pick off.  And thus far, Democrats have had limited success in recruiting 
top-tier challengers to run against potentially vulnerable GOP incumbents (Cook 2006).


The House Forecasting Model: Combining National Conditions and Strategic Politicians

	Jacobson and Kernell (1983) have argued that the effects of national conditions on 
congressional elections are mediated by the actions of "strategic politicians."  When 
national political conditions appear to favor one party over the other, the advantaged 
party is generally more successful at convincing its incumbents to run for reelection, 
thereby limiting the opposition party's open seat opportunities, and at recruiting 
politically experienced challengers who are capable of raising the large sums of money 
needed to wage competitive campaigns.1
  
	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.2   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).  
Results

	The results in Tables 1 show that the model does a very good job of explaining the 
outcomes of past House elections—all of the independent variables except the percentage 
of Republican seats in the previous Congress have statistically significant effects and the 
model explains 90 percent of the variation in the results of House elections since World 
War II.  Even after controlling for presidential approval and the actions of strategic 
politicians, the generic vote variable has a substantial impact on the outcomes of House 
elections: the current 10 point Democratic advantage in the generic vote translates into a 
pickup of approximately nine seats in the House with all other independent variables held 
constant.  



	We can use the results in Table 1 to make conditional predictions about the 
outcome of the 2006 House elections.  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.   
Forecasting the Senate Elections

	What about the Senate?  There, Democrats face a more daunting challenge than in 
the House of Representatives.  Democrats need a pickup of six seats to take control of the 
Senate.  Since World War II, the average Senate seat loss for the president's party in 
midterm elections is only three seats.  Even when the president's approval rating is below 
50 percent, the average seat loss is only five seats.  Moreover, the Democrats' task is 
made even more difficult by the fact that only 15 of the 33 Senate seats being contested 
in 2006 are currently held by Republicans.
  
	Table 2 displays the estimates for the Senate forecasting model.  The dependent 
variable in this model is the change in the number of Republican Senate seats.  The 
independent variables are the number of Republican seats at stake in the election (a 
measure of exposure), dummy variables for Republican and Democratic midterm 
elections, net presidential approval in early September, and the difference between the 
Republican and Democratic percentage of the generic vote in early September.  Variables 
measuring the actions of strategic politicians are not included in the Senate model 
because data on challenger quality is not available for Senate elections and relative 
numbers of Republican and Democratic open seats had no impact on the outcomes of 
Senate elections when it was added to the model.  



	The results in Table 2 show that the Senate forecasting model is not nearly as 
accurate as the House forecasting model, explaining only about two-thirds of the variance 
in the outcomes of Senate elections since World War II.  This is not surprising since the 
model does not include any variables measuring the actions of strategic politicians.  
Moreover, the outcomes of Senate elections are probably influenced more by chance 
factors because there are so few contests in each election.
  
	Despite the limitations of the Senate model, however, the results indicate that three 
of the five independent variables have significant effects.  In the Senate model, in 
contrast to the House model, seat exposure is the single strongest predictor of outcomes.  
This is consistent with the results of previous models of Senate election outcomes 
(Abramowitz and Segal 1986).   According to the results in Table 1, for every additional 
seat that the Republican Party has to defend in a Senate election, it loses an additional 0.8 
seats.
  
	While the effects of the presidential approval variable are not quite significant at the 
.05 level, the generic vote variables does have a statistically significant, and substantively 
important, impact on the outcomes of Senate elections despite the fact that the question 
asks about voting in House elections.  The results in Table 1 indicate that a Democratic 
advantage of 10 points on the generic vote question would produce a Democratic pickup 
of about two seats in the Senate with all other independent variables held constant.

	We can use the results in Table 2 to make conditional predictions about the 
outcome of the 2006 Senate elections.  Based on a net approval rating for President Bush 
of -20 and a Democratic advantage of 10 points in the generic vote, the model predicts a 
Democratic gain of 2.4 seats in the 2006 Senate elections.  The main reason why the 
predicted Democratic gain is not larger is that only 15 Republican seats are being 
contested this year.

  
Conclusions

	Both national conditions and the actions of strategic politicians influence the 
outcomes of U.S. House elections.  President Bush's low approval ratings and especially 
the large advantage that Democrats currently enjoy in the generic vote suggest that 
Democrats have a serious chance to regain control of the House of Representatives in 
November.  However, the size of Democratic gains in the House will depend on how 
successful Democrats are in recruiting quality challengers to run against potentially 
vulnerable Republican incumbents as well as national political conditions.  Democratic 
gains are also likely in the Senate but it will be difficult for Democrats to pick up the six 
seats that they need to take control of the upper chamber because Republicans are 
defending only 15 seats.


  

1 For additional evidence concerning the strategic politicians theory, see Jacobson (1986), Abramowitz 
(1984), Lublin (1994), and Born (1986).

2 Data on presidential approval and the generic vote were taken from the first Gallup Poll after Labor Day 
in which these questions were asked.  In a few cases, polls conducted during either August or October were 
used.  Net presidential approval was recoded so that higher scores favored Republicans and lower scores 
favored Democrats.



References

Abramowitz, Alan I.  1984.  National Issues, Strategic Politicians, and Voting Behavior 
in the 1980 and 1982 Congressional Elections.  American Journal of Political 
Science, 28 (4): 710-721.

Abramowitz, Alan I., and Jeffrey A. Segal.  1986.  Determinants of the Outcomes of U.S. 
Senate Elections. Journal of Politics, 48 (2): 433-439.

Born, Richard.  1986.  Strategic Politicians and Unresponsive Voters.  American Political 
Science Review, 80 (2): 599-612.

Cook, Charlie.  2006.  A Shortage of Top-Tier Challengers.  National Journal Online: 
www.nationaljournal.com.  April 1.

Jacobson, Gary C.  1989.  Strategic Politicians and the Dynamics of U.S. House 
Elections, 1946-86.  American Political Science Review, 89 (3): 773-793.

Jacobson, Gary C., and Samuel Kernell.  1983.  Strategy and Choice in Congressional 
Elections.  New Haven: Yale University Press.

Lublin, David I.  1994.  Quality, Not Quantity: Strategic Politicians in U.S. Senate 
Elections, 1952-1990.  Journal of Politics, 56 (1): 228-241.

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