And congressional re-districting has a bit to do with this as well. I'd argue the country -- in the main, if you distance yourself from all the rhetoric -- is divided fairly evenly between the two parties, with independents/moderates/those in the middle swaying to and fro from party and candidate, depending on the year and circumstances. Yet this isn't really reflected in our 435 congressional seats, more and more of which are drawn to the benefit of one party or the other. (In Wisconsin, the Madison-based congressional seat where I live once elected a Republican not that long ago, and re-elected him a few times, because the right kind of GOPer could win here. Now, even in what appears to be an incredibly good year to run as a Republican, the GOP candidate here has no chance against one of the more liberal members of Congress, because the seat now is so skewed toward the Dems).
Overall, it can often balance out in the end -- Congress has gone from Dem control to GOP control back to Dem control and now back to GOP control after tonight, all in the space of two decades (1992-2010). That's not necessarily an inaccurate reflection of the country. But the rigged system of congressional seats leads to candidates who play to their party's extreme tendencies, and thus political rhetoric often gets inflamed -- and (I'd argue) more importantly the willingness of the two sides to work together is often damaged. More and more of these folks just don't seem to like each other, or respect each other's ideas (or seem willing to work with the other side, fearing repercussions back home), and that's not a good thing in the long run, I'd suggest.