Notably, the expanding sectors of the population are much more progressive and pro-Democratic than their aging and white counterparts. Mrs. Clinton defeated Mr. Trump by nearly 20 points among voters under 30, and the anti-Republican tilt of that demographic was even more pronounced in 2018, when 67 percent of them voted Democratic, 35 points more than the number who voted Republican. As for Latinos, nearly two-thirds of that population consistently vote Democratic.

The implications of these developments are most significant in the specific states where the election will be most fiercely fought. In Michigan and Wisconsin, which were decided in 2016 by roughly 11,000 and 22,700 votes respectively, close to a million young people have since turned 18. Beyond the Midwestern trio of states, the demographic revolution has even more transformative potential. Mr. Trump won Arizona, for example, by 91,000 votes, and 160,000 Latinos have turned 18 in that state since then.