For Donald Trump to win the 2016 election, he needed to win states that Mitt Romney lost four years ago. To the surprise of many (including yours truly), Trump did so, winning several states that had voted Democratic for decades.
For the most part, counties don't flip that often. A plurality of counties have voted Republican consistently since at least 1980; 600 have voted Republican since 2000. Most of these counties are large, rural areas that have smaller populations. About 31.4 million voted this year in the 1,183 counties that have voted for the Republican since 1980. In the 142 counties that have voted for the Democrat each time since 1980, 15.9 million people voted this year.
What's important this year is the geography of where those counties flipped.
Most of the solidly blue counties stretch along the East Coast, down through the Deep South and across the southern border to the Pacific Coast. Many of the slightly-lighter-red counties across the middle of the country switched with the 2000 election. The most-red ones have been voting Republican since well before that.
It's the light-red counties that were important this year — the upper Midwest counties that flipped from President Obama in 2012 to Trump.
There's a clot in the overlap of Iowa and Wisconsin that helped move both of those counties into the win column for Trump.
Clinton's gains were mostly places that didn't do her any good.
She picked up wins in several Utah counties (going on to lose the state) and in California's normally conservative Orange County (helping her win a state she was already going to win by a wide margin).
There were also 81 counties that exhibited a particularly weird voting pattern, backing George W. Bush in 2004, Obama in 2008 and 2012 and then Trump in 2016. Call them the BOOT counties.
Many were in the same region noted above: Michigan, Wisconsin, Iowa.
In Pennsylvania, another state Trump won, he didn't need to flip counties to do so. The state is a good example of the urban-rural split that this map captures: pockets of Democratic votes that, because of density, can tip the state. In Pennsylvania, Trump did better in the rural areas and in suburban counties to win the presidency.
The question he will face is how many of those counties he can hold in 2020.