I think that you need to step back and take a look at who are and are not extremists. That there are 2 parties doesn't mean that they are necessarily equidistant from the "reasonable center". For example, the notion that someone who is a mainline protestant, in a long term marriage with kids, powerfully supports education as something everyone should strive for, who is personally vested in the wealth of the anation and who "supports the sacrifice of our brave men and women" in the military as someone who is "far left", simply boggles the mind.
_I'm_ not on the far left and I am well to the left of Obama.
OTOH, mainline GOP leadership includes the likes of Sam Brownback who believes in talking snaks and other bibilical inerrancy. It includes individuals who insist on the USA's preemptive right to attack any nation it deems a potential threat, and it includes those who believe the answer to the current economic crisis is further deregulation.
Now its true that there is roughly 30% of the population that agrees with these views, but that does not mean that the Dems only have 30% support as well. Theirs is actually quite a bit more - 40% core supporters and closer to 55% if you include leaners.
We also know that 16% of the population is well to the left of the Dems. So if you add up the numbers, the "extremes" of political thought are NOT being represented by Dems vs GOP. Instead its GOP vs Greens, with the Dems BEING that "rational middle".
Me? I'm a Progressive Green. And yeah, I consider the Dems too conservative for my tastes.