A random thought from a psychologist: If it seems that our country's political discourse has grown increasingly polarized, intractable, and hostile, blame the Soviet Union for it.
Specifically, blame the Soviets for collapsing and depriving us of a meaningful enemy. If we have a common enemy, a common repository for our instinctual aggression and hostility, then whatever differences we have amongst ourselves will pale in comparison and be kept within the realm of manageable political disagreements.
But each of us has 'X' amount of hostility, aggression, and need for dominance within ourselves, and that holds true for our elected officials, opinion leaders, and everyone else who is making decisions or even simply involved in politics through self-selection. Since they don't have the Soviets to rally against, their aggression has to find an outlet elsewhere in the form of conflict.
Thus the viciousness with which we aim rhetoric at people little more threatening than common criminals (now re-named "terrorists") and the venomous state of internal political discourse. There must be conflict, there must be aggression, there must be a struggle for dominance -- that is built in to being human. Freud calls it the "id" and Jung calls it the need for the "Other." So since we have no external enemies against whom we may direct these aggressive impulses, we find new enemies and the only sources of conflict visible are internal disagreements. Thus, liberals and conservatives scarcely even recognize one another as being loyal to America anymore.
I'm not entirely sure I buy it. Europe has nasty politics, but Europeans for the most part recognize it as "just politics" and maintain national identity and cohesiveness. Europeans are just as human as the rest of us, and there isn't a lot of political expression of aggression within Europe, either external or internal, that compares to what can be seen in the U.S. Consider the leaders of Europe's largest national economies -- Angela Merkel, Gordon Brown, Dmitriy Medvedev, Nicholas Sarkozy, and Silvio Berlusconi. Of them, only Berlusconi has any discernibly aggressive qualities in his public personality, which in his case manifest not in fiery rhetoric but rather in a mercurial pattern of sexual excess and casual corruption. The others are dull technocrats. And before you protest that it's Putin, not Medvedev, running the show in Russia, recall that Putin has his technocratic side as well (although I'll concede he demonstrates more aggression than Medvedev in his public persona).
Still, it's an interesting idea to chew on for a while.