I've long believed that the Democratic party holds the American center, and as moderate Republicans are replaced by far right loonies, we will only solidify our position as the centrist party.
I don't think either of you hold the center and before you call the Republicans a bunch of loonies I would caution you to not throw stones in a glass house. As one who is independent of the two party apparatus and therefore not prone to partisanship as you are, I can assure you that both of your houses have your share of crackpots. Americans overall are frustrated with both parties.
As Markwarner08 and others have pointed out, for all the obnoxious noise that comes from the left, the democrats are more centerist in their policies than the republicans are. If you compare us to most other industrialized nations, there isn't even really a left in American politics: there are certainly leftist pressure groups, but very few real leftist politicians aside from Dennis Kuicinich, a few members of the Congressional Black Caucus, and a few others.