If one race was this close you might argue that's a coincidence. However there are so many elections that decide the balance of power that come to within a few votes. And they just happen to turn out in the Dems' favor.

I'd actually be interested to get a statistical breakdown of these types of races. I'd guess that they lean more towards "establishment uniparty candidate" vs. specifically "Democrat". But it would be nice to be able to say, "yeah, it seems that somehow Democrats are just such lucky bastards that they somehow manage to win 90% of close races."