The last Gallup pre-election poll puts approval of President Bush at 38% (and disapproval at 56%.) As the figure above shows, this is the second lowest approval at midterm of any president since World War II. In previous years, such low approval would be expected to lead to major losses in congress. Whether due to redistricting or something else, few forecasts anticipate losses of the magnitude such an approval rating alone would suggest. Current estimates range from the 20s to the high 30s, with a few above and (even fewer) below that range.
Back in May I wrote about approval and midterms and estimated a forecast of approval in the last pre-election poll based on approval in May. The plot below is from that post, with the actual approval rating added. Not bad for predicting the future more than 5 months out.