First, geographic mobility in the United States has become very class-dependent. Once upon a time, lower-income people were willing to pull up stakes and move to places with greater opportunity — think of the people who fled the Dust Bowl for California in the 1930s, or those who took the “Hillbilly Highway” out of Appalachia to work in Midwestern factories, or Southern blacks on the Great Migration. In recent decades, though, internal migration has slowed sharply, and the people who are most likely to move for better opportunities are the highly educated.
Second, higher levels of education are increasingly correlated with voting Democratic. This has been most starkly on display in the 2016 election, as polls suggest that Donald J. Trump may be the first Republican in 60 years to not win a majority of white voters with college degrees, even as he holds his own among white voters without degrees. But the trend of increasing Democratic identification among college graduates, and increasing Republican identification among non-graduates, was underway before Mr. Trump arrived on the scene. Today, Democrats hold a 12-point edge in party identification among those with a college degree or more. In 2004, the parties were even on that score.
Finally, in the United States the economic gap between the wealthiest cities and the rest of the country has grown considerably. The internet was supposed to allow wealth to spread out, since we could be connected anywhere — but the opposite has happened. Per capita income in the District of Columbia has gone from 29 percent above the United States average in 1980 to 68 percent in 2013; in the Bay Area, from 50 percent above to 88 percent; in New York City, from 80 percent above to 172 percent. Cities like New York, San Francisco, Seattle and Boston exert a strong pull on mobile, highly educated, Democratic-leaning voters, while at the same time stirring resentment in the less prosperous areas those voters leave behind. And these economically dominant cities tend to be in deep-blue states.