The Refragmentation
"The Big Sort" by Bill Bishop is a well researched book that explains the political fracturing of America.<p>The topic is explains is expressed visually in these two maps, which shows county election results from 1976 to 2004.   
<a href="http://www.thebigsort.com/maps.php" rel="nofollow">http://www.thebigsort.com/maps.php</a><p>One of the point is that due to economy, mobility and choice, Americans decide to settle where people are like them the most, and that this results in deeper political boundaries, especially in the context of our electoral system.<p>One takeaway is that individually we are not all different, and a single Democrat and Republican in a room would likely come up with compromises on differing issues.  However when on Democrat/Republican is in a group of many like themselves, they tend to take the most extreme view, on average.<p>The other is that there used to be more friendly relationships between senior senators on both sides of the aisle.  They would gather regularly to have a few cocktails even if they have strongly differing views.  These bonds made it possible for them to work as a bi-partisan team to get legislation passed.  These relationships no longer exist the way they used to, and many politicians are much more transient in the time they spend in DC.