Predicting and Dissecting the Seats-Votes Curve in the 2006 U.S. House Election

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The Democrats' victory in the 2006 election has been compared to the Republicans' in 2004. But the Democrats actually did a lot better in terms of the vote. The Democrats received 54.8% of the average district vote for the two parties in 2006, whereas the Republicans only averaged 51.6% in 1994. The 2006 outcome for the Democrats is comparable to their typical vote shares as the majority party in the decades preceding the 1994 realignment. Nevertheless, the size of the Democrats' victory in the 2006 House elections has obscured the sizable structural disadvantages they faced heading into the elections. In this paper we document the advantages the Republicans had, examine how and to what extent the Democrats overcame it, and offer predictions as to whether the results of the 2006 election leveled the electoral playing field for 2008. Our calculations showed that the Democrats needed at least 52% of the vote to have an even chance of taking control of the House of Representatives.

Prior to the election 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 found that the Democrats in 2006 were always destined to receive fewer seats than their corresponding average vote share. They were able to gain control of the House by winning the largest average district vote by either party since 1990. Has the 2006 election removed the Republicans' structural advantages? While Republicans continue to win more close races, a preliminary analysis of the 2008 election suggests that the switch in incumbency advantage from the Republicans to the Democrats may nevertheless level the electoral playing field.