The "generic ballot" question is a long-standing way of looking at congressional elections:
"If the elections for Congress were being held today, which party's candidate would you vote for in your congressional district?"
Like presidential approval, it's a question different pollsters all ask roughly the same way so it can be compared.
Democrats always lead on this question. Even in years when we actually lose some seats we still usually lead on the generic ballot question because there are more registered Dems nationally and there are more highly concentrated Democratic districts.
The old rule of thumb was that Dems were supposed to lead by 6-8% for it to be break-even.
So the fact that we are behind in the
two latest generic ballot polls is noteworthy.
USA Today/Gallup Poll. March 26-28 Dem 45% Rep 46%
CNN/Opinion Research Corporation Poll. March 25-28, Dem 45% Rep 49%
(Also down 44-47 in Gallup weekly tracking on the question but since they conduct the USA Today poll I only said two, not three)
http://pollingreport.com/2010.htm