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Thursday, October 19, 2006

"It's Not Easy Being a Democrat"

Andrew Gelman of the Statistical Modeling, Causal Inference, and Social Science blog at Columbia University sends this follow-up to Krugman's analysis of the odds that Democrats will retake the House of Representatives. Krugman contends that if Democrats can get enough votes to overcome the Republican "levee," (see the graph in Krugman's analysis) they will win a flood of seats, but it is not certain they can get enough votes for this happen. This analysis, which differs from Krugman's in that it allows for incumbency and for year-to-year variation in the relative positions of districts, says it won't be easy. Their estimate is that "Democrats will need ... 55% of the vote to have a 90% chance of winning the House":

Seeking 50% of Seats, Needing More than 50% of Votes: Predicting the Seats-Votes Curve in the 2006 Elections, by Jonathan P. Kastellec, Andrew Gelman, and Jamie P. Chandler, October 14, 2006: Abstract After their stunning loss of both houses of Congress in 1994, the Democrats have averaged over 50% of the vote in Congressional races in every year except 2002, yet they have not regained control of the House. The same is true with the Senate: in the last three elections (in which 100 senators were elected), Democratic candidates earned three million more votes than Republican candidates, yet they are outnumbered by Republicans in the Senate as well. 2006 is looking better for the Democrats, but our calculations show that they need to average at least 52% of the vote (which is more than either party has received since 1992) to have an even chance of taking control of the House of Representatives. Why are things so tough? Looking at the 2004 election, the Democrats won their victories with an average of 69% of the vote, while the Republicans averaged 65% in their contests, thus “wasting” fewer votes. More formally, we estimated the seats-votes curve for 2006 by constructing a model to predict the 2006 election from 2004, and then validating the method by applying it to previous elections (predicting 2004 from 2002, and so forth). We predict that the Democrats will need 49% of the average vote to have a 10% chance, 52% of the vote to have an even chance, and 55% of the vote to have a 90% chance of winning the House. The Democrats might be able to do it, but it won’t be easy.

Gelman101506
Click on figure for larger version

Here's Andrew's discussion of their paper:

Seeking 50% of Seats, Needing More than 50% of Votes, by Andrew Gelman: It's not easy being a Democrat. After their stunning loss of both houses of Congress in 1994, the Democrats have averaged over 50% of the vote in Congressional races in every year except 2002, yet they have not regained control of the House. The same is true with the Senate: in the last three elections (during which 100 senators were elected), Democratic candidates have earned three million more votes than Republican candidates, yet they are outnumbered by Republicans in the Senate as well. 2006 is looking better for the Democrats, but our calculations show that they need to average at least 52% of the vote (which is more than either party has received since 1992) to have an even chance of taking control of the House of Representatives.

Why are things so tough? Looking at the 2004 election, the Democrats won their victories with an average of 69% of the vote, while the Republicans averaged 65% in their contests, thus ''wasting'' fewer votes. The Republicans won 47 races with less than 60% of the vote; the Democrats only 28. Many Democrats are in districts where they win overwhelmingly, while many Republicans are winning the close races--with the benefit of incumbency and, in some cases, favorable redistricting.

The accompanying chart (larger version here) shows the Democrats' share of the Congressional vote over the past few decades, along with what we estimate they need to have a 10%, 50%, and 90% chance of winning the crucial 218 seats in the House of Representatives. We performed the calculation by constructing a model to predict the 2006 election from 2004, and then validating the method by applying it to previous elections (predicting 2004 from 2002, and so forth). We predict that the Democrats will need 49% of the average vote to have a 10% chance, 52% of the vote to have an even chance, and 55% of the vote to have a 90% chance of winning the House. The Democrats might be able to do it, but it won't be easy. ...

P.S. After we wrote this article (and the above summary), we were pointed to some related discussions by Paul Krugman (see links/discussions from Mark Thoma and Kevin Drum) and Eric Alterman. They do their calculations using uniform partisan swing whereas we allow for variation among districts in swings, but the general results are the same.

"It won't be easy."

    Posted by on Thursday, October 19, 2006 at 10:12 AM in Economics, Politics | Permalink  TrackBack (0)  Comments (21)

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