Obama's approval rating is 45%--Clinton's was 46% on the eve of the disasterous 1994 midterms. Historically, according to this WSJ blog I just read (how's that for a citation), generic congressional poll differences have done a good job pr...edicting midterms, the more difference nationally, the more seats won/lost. Worse for the Democrats, as the election draws nearer, Gallup will switch from polling registered voters to likely voters, which is expected to bump GOP numbers up even higher. Finally, just for gee-whiz sake, the reason all this is true is that older white voters make up a higher percentage of midterm voters than their percentage in general elections. That may comfort Democrats considerably in 20 years but makes things pretty dismal now. And in 20 years, there will be a genuine major party to the left of the Democrats anyway.
On the other hand, if the GOP does win control of both houses in November, I wouldn't be surprised if Obama's approval ratings shot up through the roof by March or April.