Well, the center from the viewpoint of the American voting public. For whatever reason, we split almost exactly 50:50 in both of the last two elections--as you know.

Also note that despite the desire of some of us for a truly liberal Democratic party, the fact is that the Dems don't support a whole range of things that are supported by conservatives in other countries, like Canada. Stuff like single payer health, strong environmental policies (remember Kyoto was rejected 99:1 in the Senate), secular politics, etc., that are routine in Europe are not even under consideration here. America is simply not on the same page as Europe in many ways: What's the "center" here is off in the weeds in Europe, and vice versa.