The Democrats did not run on any kind of national theme. They did not win a contest of ideas. This is in sharp contrast with the '94 Republicans who ran on a party plank. See The default majority By Rich Lowry:
"This is the kind of choice Democrats were able to avoid in November when they became a default majority -- a majority elected not for what it stood for, but for what it was not. Eventually, Democrats will have to move beyond their default position if they want a record of substantive accomplishment after the pageantry and symbolism of the 100 hours have passed."