Donald Trump owes his victory in the Electoral College to three states he won by the smallest number of votes: Pennsylvania, Wisconsin, and Michigan. So it’s fair to say that the 2016 presidential election was decided by about 100,000 votes out of than 120 million ballots cast. According to the latest tallies, Trump won Pennsylvania by 1.1 percentage points (68,236 votes), Wisconsin by 0.9 points (27,257 votes), Michigan by 0.2 points (11,837 votes). If Clinton had won all three states, she would have won the Electoral College 278 to 260. She fell short in all three, of course, and that’s why we now getting accustomed to the reality of President-elect Donald J. Trump.

Trump’s victory in these three states was a big shift from 2012, when Obama won Michigan by 9.5 points, Wisconsin by 6.7 points, and Pennsylvania by 5.2 points. Although the national vote swung only about 3 points toward GOP in 2016 (leaving Hillary Clinton as the narrow winner of the popular vote), these three states swung by 6 to 10 points toward the Republican presidential nominee.

So what accounts for the swing? A close look at the exit polls reveals a slightly different story in each state, but most of the change is due to the fact that Clinton performed much worse than Obama did among middle- and low-income voters.