Loudoun County, Va., and Jefferson County, W.Va., share bucolic countryside and a 14-mile border. But there are two big differences between them that capture something fundamental about our age.

Economically, Loudoun County is humming from the technology boom in Washington’s suburbs, with the number of businesses rising 49 percent from 2005 to 2015. But on the other side of that border, Jefferson County doesn’t have the same economic dynamism: The number of businesses in the county fell 11 percent in the same period, according to census data.

The two counties have diverged politically, too. Back in 2004, both favored George W. Bush for president by a similar, moderate margin. In 2016, Hillary Clinton won Loudoun by 17 percentage points; Jefferson favored Donald J. Trump by 15 points.

It is a chasm you can find nationwide, according to calculations by Jim Kessler of the think tank Third Way. In 2004, there was just a two-percentage-point gap in how the presidential candidates fared between counties that had gained businesses versus those that had lost them in the preceding decade. In 2016, that was a 15-point gap, with the places that had lost business favoring Mr. Trump.