In 2012, something unusual happened. The wealthiest 4 percent of voting-age Americans, by a narrow plurality, supported a Democrat for president.
This hadn't happened since 1964. Before that, it hadn't happened since possibly the 1880s (scientific survey data for back then is, sadly, nonexistent).
There are a few things we know have become very strong predictors of voting Democratic. One is that nonwhite people tend to support Democrats at higher rates than white people do. Another is that the highly educated have become much more liberal over time, making educational attainment a better predictor of voting for a Democrat.
And over time, the top 4 percent has become much more diverse and much more highly educated.
Let's look at the diversity first, because the story here is pretty straightforward. In 1952, the top 4 percent of wealthiest Americans were entirely white. By 2012, 75 percent of the top 4 percent self-identified as white. So the wealthiest Americans have become more diverse over time.
The top 4 percent have also become much more highly educated over time, as have Americans overall. Again, given that there is a strong relationship between educational attainment and liberal values, especially in recent years, it might not be surprising that wealthy elites have become more Democratic-leaning as they've become more highly educated.
As Bonica and Rosenthal write, "Changes in partisanship could well reflect changes from a manufacturing and extraction economy to a technology and information economy — Silicon Valley and Hollywood are generous to Democrats." Even before Trump locked up the GOP nomination, these trends were continuing in the 2016 election.