The generic Congressional vote has closed from a consistent double-digit advantage for Democrats as recently as mid-summer to a mere 3.6 lead in the current RCP average. That's probably somewhat distorted by a recent wacky Gallup poll which showed an implausible +5 Republicans edge, but two other credible polls in the past week put Democrats ahead by a mere 3 points and 5 points. A month ago, by contrast, an AP-Ipsos poll had Democrats ahead by 18 points.
This is tough to explain, but I'd guess that it's mostly about energy. Congressional Republicans mounted a skillful summer PR offensive that forced Nancy Pelosi to back down from her once-adamant opposition to new offshore oil drilling, and these numbers may reflect that the public took note.
In any event, it's worrisome for Democrats who are wondering whether the newly-tied presidential race is a function of Palin mania or some deeper shift in voter attitudes.