The Dims 'won' the Congress in '06 ONLY because the Pubes din't haul out their whole arsenal of reliable, time-tested, heavy, vote-stealing artillery.
They--the GOPhux--didn't WANT to 'win' in Congress. They DID want to be able to shift blame onto the Democrats for the Busheviki excesses over the previous 6 years and inoculation against the the forthcoming two years.
So, the Pubes more or less threw the '06 'election' because they needed to be able to dump some share of the responsibility for the Bushevik "Blunders" (they're not, really; those 'blunders' are actually the Bushevik 'accomplishments; but that's for another time) on the Dims.
The Dim margin of 'victory' provided that, despite the appearance of majorities in both Houses, the Pubes retained effective control in both. It worked just about perfectly, seems to me.
Wonderful how the Dims stepped Right UP, innit?!