A lot of the polarization has to do with gerrymandering. With so many districts completely in one party, candidates have to distinguish themselves in the primary where it often pays to be as far to the left/right as possible.
That doesn't explain the Senate.
The Senate...most of your moderates seem to be Red Senators in Blue States (Save Santorum), or Blue Senators in Red States (ie Nelson)...and they aren't that sizable. It seems, that the extremism is driven by the fact that you've got crazy Red Senators safely seated in red states, along with Blue Crazies in blue states. I'm sure you want to say that the Reds are crazier than the blues, but I'm not in the mood for partisan drivel.
The only way you get those crazies out is through an act of sheer stupidity on their part (Ie Burns, Allen...maybe Menendez[who is more corrupt than crazy]).
I'm hard pressed, other than Santorum, to name a crazy senator from a state his/her party lost in the 2000 or 2004 election.
