I thought this too until I looked at US presidential electoral maps for the past century, and then ran across a study of political division in the US that showe

I thought this too until I looked at US presidential electoral maps for the past century, and then ran across a study of political division in the US that showed a dramatic increase beginning around 1980. (Don't have a link handy unfortunately.) It seems the degree and nature of division in the US really has changed in a fundamental way.