I also think that the system of party primary elections by popular vote is a big part of the problem. To win the Republican primary you have to be more "Republican" than your opponents. Ditto for the Democratic primary. Then you end up with candidates at the far ends of the spectrum instead of more centrist ones.

In congress this means a complete inability to compromise, resulting in the current stalemate. In the presidency, you end up with someone who thinks of his opponents as criminals deserving prosecution.