From the piece:
Starting before the Civil War era, America's political dividing lines were drawn along state and regional borders. Cities and the then-extensive rural areas shared a worldview North and South of the Mason-Dixon line. While there was always tension within states, they were bound by a common politics. The city of Charleston, for example, was as rabidly anti-North as some inland plantation areas. Economic engines, ways of life, and moral philosophies changed at the 36th parallel, where the North began.
Today, that divide has vanished.