A few years ago, on my way from eastern Oregon to Salt Lake City, I pulled off I-84 in Boise, Idaho, a place I knew only from a Lynyrd Skynyrd lyric. I strolled downtown and, in a farmers’ market smelling of lemon sage and lavender, found the best artisanal granola bars west of Burlington, Vermont. As I scanned the pale young crowd and noted the vendors’ earnestly precious names (Hoot ’n’ Holler Urban Farm, Blue Feather Bakery), it occurred to me how much the scene resembled a Saturday morning at Smorgasburg, or at Ferry Plaza in San Francisco, or in Boulder, Sante Fe, or Portland, Maine. The vibe was relaxed, prosperous, and gently urban. Around that time, the real-estate blog Estately ranked Boise fifth on its list of best cities for conservatives: “The tech jobs are plentiful, the Mormonism abundant, there are no restrictions on assault weapons, and there’s plenty of art, music, and theater.” And yet, like most red-state cities, Idaho’s capital is remarkably short on conservatives. Last November, while Hillary Clinton mustered only 27.5 percent of the statewide vote, she hit north of 75 percent in some of Boise’s urban precincts. Politically, the city might as well be on a different planet from towns that lie a couple of exits away.
And this is true in more than just America’s twee precincts. More than ever, urbanites share a common affiliation, whether they are in Idaho or Massachusetts, whether they’ve built up earthly treasures or scrape by on minimum wage. In presidential elections, votes are tallied by county and grouped by state, but zoom in closer and you see that Democrats cluster like blue inkblots on a mostly red map and that many jurisdictions are sharply divided. In the 2016 election, virtually every large urban center and many small ones — white Boise and majority-black Baltimore, wealthy San Francisco and beaten-down Detroit, sprawling sunwashed megalopoli and shrinking union strongholds — rejected the man who became president, often by yawning margins. The density that is one of the defining characteristics of cities forces encounters that, more and more, seem to strengthen Democratic principles — and separates urban dwellers from their rural cousins.
While cities like New York and Seattle have always been liberal, others have converted much more recently. After World War II, San Francisco was up for grabs, voting Republican in 1952 and 1956, after which its Democratic tilt increased from year to year. Philadelphia was a Republican bastion until 1951, when a new charter combined with a corruption scandal to demolish the political machine. Ward leaders and local bosses switched their allegiances just as middle-class whites were fleeing to the suburbs, leaving the heavily black central city to the party of civil rights. […]