Most of the change, though, occurred not around the big cities — where Democratic candidates have only so many votes left to pick up — but far outside them. Just 15 counties tilted by more than five percentage points in favor of Mrs. Clinton relative to how they voted in 2012. By contrast, 1,826 moved by at least that much away from the Democratic candidate.

The counties that swung the most drastically toward Mr. Trump, by 15 points or more, were nearly all in the Midwest. That abrupt shift was probably driven by numerous factors that are hard to untangle: weak economic prospects; Mrs. Clinton’s lack of attention to those places on the campaign trail; Mr. Trump’s xenophobic message to voters anxious about change.

But the widening political divergence between cities and small-town America also reflects a growing alienation between the two groups, and a sense — perhaps accurate — that their fates are not connected.

The University of Wisconsin political scientist Katherine J. Cramer, the author of “The Politics of Resentment,” described what this looked like during years of field research in Wisconsin in an insightful interview with Jeff Guo at The Washington Post. The people she met across a state that Mrs. Clinton ultimately lost felt deeply disrespected (and suspicious of a white-collar academic from uber-blue Madison). “They would say, ‘The real kicker is that people in the city don’t understand us,’ ” Ms. Cramer said. “ ‘They don’t understand what rural life is like, what’s important to us and what challenges that we’re facing. They think we’re a bunch of redneck racists.’ ”

Cities, for their part, are easily branded with some dissonance as embodying either professional elites or poor people who don’t deserve benefits (thus both Madison and Milwaukee, two very different places, come in for equal resentment within Wisconsin). Many of the young Democratic voters who live in blue cities like these, as Alec MacGillis has noted, have gravitated away from redder parts of the country from which they felt alienated. “There’s just nothing to do in Ohio,” lamented one voter who grew up there but now lives in Los Angeles. “The jobs are limited, but it’s not just the jobs and the industries that are in Ohio, it’s the mind-set that I didn’t gravitate to.”