Throw in the GOP's "culture of corruption," a cooling economy (with the housing bubble poised to unwind and the Fed raising rates, it isn't likely to get much better, anyway) and the normal historical pattern for second-term, mid-term elections, and next year might be an opportunity even the Dems can't kick away -- if, that is, they can work out a viable compromise between the "50-state" strategy of challenging the Republicans in every district (mission impossible) and an overly cautious focus on just a handful of "competitive" districts (business as usual).
On the other hand, given where things stand now, if the Dems don't do well next year, either Shrub and company will have pulled off a miracle in Iraq, the economy will have entered supply-side heaven, or we'll know for certain that we're effectively living in a one-party state.
2006 will mark 4 years of unified government, by the way, with all three branches of the Federal government being held by one party. 2006 represents one chance to return some diversity back to one of those branches.
I also feel it will probably be one of the few chances that we have to see if the rot I and many others sense is real.