As I wrote a couple of weeks ago, the Republicans need something like a 7% swing in the national vote to take back the House of Representatives in 2010.

From Erikson, Bafumi, and Wlezien, here is a graph predicting the Democratic party’s vote share in midterm elections, given their support in a generic party ballot from polls taken during the 300 days before the election:

The higher line in each graph (in red) corresponds to elections where the incumbent president is a Republican, and the lower line (in blue) corresponds to elections such as 2010, where the incumbent is a Democrat.