There is no center--except, perhaps, inside the Beltway. From an important opinion piece by Thomas F. Schaller in the outside-the-Beltway Baltimore Sun:

The one thing media talking heads agree upon is that the center prevails. Turn on almost any of the nation's political talk shows and pretty soon somebody will say how crucial it is for politicians to appeal to registered independents and self-described moderate voters.

They conjure for us an image of the distribution of the American electorate as that of a dromedary's single hump with a large, vital center of thoughtful citizens in the middle, flanked by a downward-sloping share of shrill, radical liberals on one side and grumbling, reactionary conservatives on the other.

The true image is that of the two-humped camel: The American electorate has for some time been bifurcating into two rather distinct camps, with fewer centrist voters.

On a panel at a Chicago convention of political scientists recently, Emory University's Alan Abramowitz explained what's happening.