Targator's excellent diary on new research about how human brains process political dissonance is worth more discussion. The main thesis is this:
Essentially, it appears as if partisans twirl the cognitive kaleidoscope until they get the conclusions they want, and then they get massively reinforced for it, with the elimination of negative emotional states and activation of positive ones. The result is that partisan beliefs are calcified, and the person can learn very little from new data.
As I noted when the diary was first posted, I fully admit that I can barely stand to listen to wingers talk. I've given that matter some serious thought and arrived at my own 'partisan' explanation: