Partisan sorting is a very recent phenomenon, and has been driven by the Southern realignment.

Past research has argued that over the last 30 years, the United States has become much more geographically polarized. Democrats and Republicans, the argument goes, are moving to different locations, creating a society in which voters are unlikely to know anyone who has an alternative perspective on politics. But how much have Americans really polarized over the last 30 years? In new research which utilizes data on presidential voting by county from 1972-2012, Corey Lang and Shanna Pearson-Merkowitz find that while partisan sorting is on the rise, it is a fairly recent phenomenon, which appears to be mainly driven by the Southern realignment and not housing decisions