Much of the political commentary since the presidential election has focused on two groups of party switchers: those who voted for Barack Obama in 2012 and Donald Trump in 2016 and those who voted for Mitt Romney in 2012 and Hillary Clinton in 2016.

Trump voters who previously voted for Mr. Obama are the subject of intense fascination because they are viewed as providing critical insights into the racial and class dynamics that helped determine the outcome of the election. On the other side, many analysts see Romney voters who flipped to Mrs. Clinton as an illustration of how the Democratic Party now survives in significant part by appealing to more upscale voters.

Frustratingly, however, these perspectives play down the importance of a crucial group of disaffected voters: those who voted for Mr. Obama in 2012 but then failed to go to the polls in 2016. Because this group is disproportionately young and black, this erasure is racially tinged.

Our analysis shows that while 9 percent of Obama 2012 voters went for Mr. Trump in 2016, 7 percent — that’s more than four million missing voters — stayed home. Three percent voted for a third-party candidate.