You're talking about an absolute center on a universal political spectrum. I'm talking about a political center that's relative to the two prevailing parties in the US. The point isn't that the Democrats moved to "the center," but that they moved to the right (to the center between where the Democrats used to be and where the Republicans would have to go to distinguish their brand), especially on economic issues. The Democrats betrayed their progressive principles, whereas the Republicans purified their social Darwinian ones.

From the far right, a moderate looks extreme, but that's a projection of the far right perspective. The only thing extreme (i.e. unusual) about Obama was his skin colour. But the two parties' shared neoliberalism can look extreme from a far left or environmentalist perspective. We have to distinguish the absolute from the relative and subjective classifications.