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.