Terrific NYT infographic -- the colored counties are those whose presidential votes were "landslides" i.e. the winner 20 percent or more above the loser. The maps look like Hot Zone or any of the virus outbreak movies -- polarization engulfs the entire population over time. More data on the site about the characteristics of the red and blue counties' populations.
If you're interested in the causes of this trend (ok, *seriously* interested) this 1971 paper by game theorist Thomas Schelling outlines the many mechanisms that make it almost inevitable. The paper is tough going, but the first couple of pages make clear that even without gerrymandering or malign intent, normal human behavior will lead in this direction unless there's some countervailing force.
What might that be? If France, which puts enormous resources into teaching children what it means to be French, is supporting Le Pen, it's hard to imagine what it will take to make the US one country. Your thoughts? --CAM