Democrats led on the congressional generic ballot for most of the summer. (A generic ballot is where the pollster simply asks whether, if the election were held today, the person being polled would vote for a Republican or Democratic congressional candidate. No name is attached to the selection.) Recent polls though all show Republicans with the lead.
The most important thing to look for in such polls is movement. The best way to gauge movement is to look at polls done by the same polling outfit. That way one knows the any change in the results are due to the changing views, not the methodology employed which would be the same in polls done by the same polling outfit. Here are the noteworthy results:
CNN Poll 7/20: Democrats lead by 4 pointsIn 2010, there was a major Republican wave. I don't think many analysts are anticipating a similar wave, but a smaller wave does appear to be forming. Whether the smaller GOP wave dissipates before it arrives the shore of the election is an open question.
CNN Poll 9/7: Republicans lead by 4 points.
Republicans improved by 8 points in just six weeks.
FoxNews Poll 8/12: Democrats lead by 7 points
FoxNews Poll 9/9: Republicans lead by 7 points
Republicans improved by 14 points in less than a month.
These generic polls can be found at RealClearPolitics.