When we ran the various possible House models last week, one of the most interesting (yet not at all surprising) results was a strong and statistically significant correlation between the gain for the opposition party and the party of the president. That is, all other things being equal (such as the president's approval rating and the generic ballot), House Republicans tended to do better against Democratic presidents than House Democrats did against Republican presidents. This was not something immediately obvious, but makes sense when you consider that a tie in the generic ballot predicts a six-seat gain for Republicans. The reason for this is primarily because of fundamental differences between presidential elections and midterm elections, which boil down to this graph:
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| Source: McDonald, Michael P. "American Voter Turnout in Historical Perspective." In Oxford Handbook of American Elections and Political Behavior, ed. Jan Leighley. Cambridge, UK: Oxford University Press, 2010. |
