The Cook Political Report mines voting and census data and finds that “in the past decade, the most significant predictor of a county’s shift in partisan preference has not been existing partisanship, or even income. It is level of educational attainment. Democrats have gained ground in places where those with at least a bachelor’s degree comprise high shares of the electorate, and have seen their fortunes fade elsewhere.”
The trend line in the nation’s 3114 counties: “Each year, the Democratic candidate performed progressively better as shares of degree-holders rose.”
This explains the strong red shift in parts of the south but not the blue shift in formerly-staunchly-red portions of Minnesota, Ohio and Wisconsin in the past election.