Why?
If the Democrats take the Congress on Republican turnovers Nancy Pelosi will completely misread the voters will. She'll believe that the country desires San Francisco values in DC and run with it. By the end of 2 years she will destroy the chances for her party to move behind a temporary gain.
I've often thought the only benefit to a Democrat takeover is that the 2008 GOP nominee gets to run against Washington.
And that would be the only benefit.