The pattern here is that the counties had been pretty consistent in their support of the Democratic candidate, like Trumbull. But in 2016, there was a sharp shift toward Trump. The outlier in this case is Elliott County, Kentucky, which had a truly staggering shift from 2012, voting nearly 47 points more Republican on net. The county is a distillation of the Democrats’ problems with blue-collar voters, going from very, very blue to very red in relatively short order.
So why did these reliably blue counties suddenly decide to flip to Trump? There’s a clear correlation between the degree to which the county’s vote choice changed over the last four years and the density in the county of Trump’s strongest demographic.