I'm sure most people here, partisans or otherwise, have thought to themselves at one point "How can a person believe the things they do", whether its conservatism, liberalism, Christianity, Islam, or any other mass movement. I've just finished reading a book for the second time called "The True Believer" by Eric Hoffer and its become sort of a Bible for me when it comes to understanding people.
Whats really great about this book in my opinion is that it is not done with much sense of superiority or judgment by the author. He doesn't claim or make it clear with his tone or language that he views himself as above or immune to any other person and merely provides an analysis on what he considers to be part of "the Human condition" and that is the psychological causes of fanaticism. He doesn't claim to have all the answers either.
He also does not, except for the cases of Communism and Nazism, judge mass movements as being good or bad. He even cites individuals such as Abraham Lincoln and Gandhi as because those who begun mass movements, and who clearly accomplished something good. Again, he's merely attempting to understand something about the nature of Man.
This book was written in 1951 and focuses a great deal on Fascism, ie Nazism in the book, and Communism of the Soviet Union. Now I do not claim that those who consider themselves conservatives, liberals, Christians, Muslims, etc to be fanatics or members of fanatical groups, there are in my opinion many similarities between the psychological reasons an individual becomes a die-hard, sometimes literal, support of a certain group, whether its true fanaticism like Nazism or something more tame such as modern day conservatism or liberalism.
Whether you read it because you think it'll tell why those Reps or Dems, Conservatives or Liberals, are they way they are or because you want to understand yourself or people better in general I think you'll find it an extremely eye opening read.