I’m not sure how I missed this before (oh, I suppose it was probably the screaming infant), but Gelman had an article in the NYTimes about the divide between red and blue America.
I like to read Gelman because he is so good at simplifying his explanation. According to this article, lower income Americans vote overwhelmingly Democrat, whereas wealthier Americans are split between Democrat and Republican. The red/blue state phenomenon is caused by skillful gerrymandering.
Of note, however, is that several of the missing states are “red” states, and it is possible that some of them could alter the analysis if they had reported exit polling data.