The answer is that though the richest counties and states tend to go for Democrats, within those places, the rich are still more likely to vote for Republicans than Democrats. It’s just that the places with lots of Democrats tend to be richer.
Take Kentucky and New York: the former is a Republican stronghold and the latter is reliably Democratic. In both states, the rich are more likely to support Republicans than are the poor. But, overall, Kentucky voters tilt toward Republicans and New York voters tilt toward Democrats—poor Kentuckians were less likely to vote for Trump than rich New Yorkers (*). Since New York is richer than Kentucky, it makes it seem like Democrats are the party of the rich.
...
The main reason Democratic states tend to be richer, even though Democratic voters are not, is that the Democrats have become the party of cities. All of the densest counties in the US vote for Democrats, most of them overwhelmingly so, and, on average, city dwellers tend to have higher incomes than rural Americans.
...
It is not the rich in the richest places that make them so Democratic, but the middle class. Working-class teachers, bus drivers, and carpenters are very likely to be Democratic in urban counties, and they make more money than the middle class in rural areas. They are the reason Democrats win in rich places without winning the rich: Democrats are the party of, among other groups, the urban middle class.
...
All of this suggests that to understand American politics you have to start with both urbanization and race. The US economy is powered by incredibly productive cities and its politics are increasingly oriented around a divide between those who live in them and those who don’t. Few divisions in America are large enough to supersede that one, and the exception is the country’s long and shameful history of racial discrimination.
Why Joe Biden will win rich places but not rich people