Andrew Gelman offers a more detailed perspective on the current generic house ballot polls and the 2010 midterms:
Bafumi, Erikson, and Wlezien’s analysis doesn’t go back before 300 days before the election, but if we take the liberty of extrapolating . . . The current state of the generic polls gives the Democrats .412/(.412+.377) = 52% of the two-party vote. Going to the graph, we see, first, that 52% for the Democrats is near historic lows (comparable to 1946, 1994, and 1998) and that the expected Democratic vote — given that their party holds the White House — is around -3%, or a 53–47 popular vote win for the Republicans.
Would 53% of the popular vote be enough for the Republicans to win a House majority? A quick look, based on my analysis with John Kastellec and Jamie Chandler of seats and votes in Congress, suggests yes.
Looked at this way, Democrats had better hope the economic situation starts improving (in the sense of conditions actually improving, rather than rate of change looking better) and that improvement starts lifting their fortunes. Alternatively, an optimistic congressional Democrat could try to take solace in the extreme unpopularity of the GOP. But I actually wouldn’t count on that. The electorate’s first choice may be for the Democrats to lose seats without John Boehner becoming Speaker, but individual voters have no way to ensure that their preferences are aggregated that way.