David Wasserman at FiveThirtyEight:
More than 61 percent of voters cast ballots in counties that gave either Clinton or Trump at least 60 percent of the major-party vote last November. That’s up from 50 percent of voters who lived in such counties in 2012 and 39 percent in 1992 — an accelerating trend that confirms that America’s political fabric, geographically, is tearing apart.
Of the nation’s 3,113 counties (or county equivalents), just 303 were decided by single-digit margins — less than 10 percent. In contrast, 1,096 counties fit that description in 1992, even though that election featured a wider national spread.1 During the same period, the number of extreme landslide counties — those decided by margins exceeding 50 percentage points — exploded from 93 to 1,196, or over a third of the nation’s counties.