Trumbull County, Ohio sits up against the Pennsylvania border in the northeastern corner of the state. Its claim to fame is that it's basically square, 25 miles to a side, split into 25 five-mile-square townships. It's just north of the more-famous city of Youngstown but shares the same broad description: Rust Belt. They still make cars in Trumbull County, down in Lordstown. Trumbull County was never terribly progressive, but it has long been the sort of working-class place that voted Democratic.

In 2012, Trumbull County backed Barack Obama's reelection by more than 22 points. This year, Trumbull went for Donald Trump by 6.4.

More remarkable than that swing, really, is the fact that Trumbull County voted for the Democratic candidate in 2008, too. And in 2004, and 2000. And in every single election back to 1976.

In all, there were 27 counties that had supported the Democratic candidate consistently for at least 40 years which switched to Trump in 2016. There were also some counties that made the other switch, backing Republicans from at least 1976 through 2012, switching to back Hillary Clinton this year. But only four counties met that standard -- and for very different reasons.

Most of the counties that had been reliably blue until 2016 were, like Trumbull, in the Rust Belt and Upper Midwest. There were a smattering across the Deep South, too, and some in the Northeast. A cluster near Seattle pops up as well.

The red counties that turned blue after decades of reliable Republican ballots are more scattered.

The pattern with these counties is pretty consistent: They were generally trending more blue over time anyway, thanks largely to shifting demographics. The most dramatic shift since 2012 among these party-flippers was in Salt Lake County, thanks to the combination of Mitt Romney's bid four years ago and broad opposition to Trump in Utah.

The counties that had been consistently voting Democratic until this year, though, look different. Here are those that showed the biggest change since 2012.

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.

Specifically, there's a clear correlation between the extent of the change since 2012 and the density of the population that is white men over the age of 25 with a high school degree or less. The more white men that fit in that group there are -- the group that was the core of Trump's support -- the more these counties swung over the last four years. (The red dot at upper right is Elliott County. The blue dot furthest to the left is Salt Lake.)

In Trumbull County, nearly 1 in 5 residents is a white man over 25 with a high school degree or less. It was one of a number of counties to which Donald Trump hoped to appeal and that he argued would be the core of his support.

To the surprise of many observers, including one who went to high school in Trumbull County and who is writing this article: He did.