Doing my daily 1 p.m. Gallup poll check (McCain's lead is shrinking), I discovered that the generic congressional poll that measures party preference, where the Democrats have been leading steadily by approximately 10 points since 2006, had shifted, giving the GOP a small deficit among registered voters and a lead in the likely voter model. It was a bit surprising, and I e-mailed a few people for responses. It turns out the poll is a bit of an outlier, or rather, it's a bit outdated. The Gallup poll was taken immediately after the conventions, when partisan preferences are hardest (Republicans were reminded why they like being Republicans). But a few days later, the magic wore off, because these polls from Rasmussen, Hotline (no link available) and Research 2000 (done for Daily Kos) all have the Democrats up 7 to 8 points, comparable to generic congressional ballots in the last election cycle.
My guess is that, as the national polls are starting to reflect, the convention bounce is wearing off and weak partisans are rethinking things. But I wouldn't be too surprised if the Democrats have less of a lead in the generic this cycle simply because of the Presidential race: voters are thinking nationally and that means by party line, as opposed to during an off-year election when they have the luxury of thinking only of their district's representative.
I contacted Ken Spain, the NRCC's press secretary, who pointed to the presidential race factor in the polling and suggested an additional explanation: The House GOP's energy-focused message over the summer months. While energy was a successful issue for Republicans this summer, I think the party leadership's refusal, thus far, to get behind the drilling/sustainable investment compromise might have taken the wind of those particular sails.