For example, in 1988 the mean level of racial resentment, on a scale of 0 to 1, was .65 for white Republicans and .61 for white Democrats. By 2016, the mean for white Republicans rose to .70, but fell for white Democrats to .41. While the proportion of racially resentful white Republicans grew only slightly, the Trump campaign’s rhetoric raised the salience of race. Democrats, by contrast, grew increasingly liberal.

Daniel J. Hopkins, author of the forthcoming article “The Activation of Prejudice and Presidential Voting: Panel Evidence from the 2016 U.S. Election,” argues that Trump “activated” racially hostile whites, using divisive language to increase his margin of victory among them.

“The marginal role of anti-black prejudice in 2016 was markedly stronger than it was in 2012, when Barack Obama was on the ballot,” Hopkins wrote in an email, adding that

Anti-black prejudice has the strongest predictive power for the 2016 elections involving Trump. I certainly don’t think that anti-Black prejudice is the entire explanation for Trump’s 2016 victory. But my evidence indicates that it was clearly an element in Trump’s electoral support, and that its role in 2016 was notably different than in prior elections.

The combustible mix of race and immigration was crucial for Trump. As Hopkins wrote,

It’s clear that Trump’s support in the 2016 primaries was concentrated among Republicans who were especially concerned about immigration and who expressed higher levels of anti-Black racial prejudice.

Among all whites, however,

It’s a different story. For them, Trump’s racially charged appeals have had polarizing impact: they pushed millions of Americans to express lower levels of anti-black prejudice and to worry more about discrimination. But those appeals also galvanized a smaller fraction of highly prejudiced Americans to express and act on that prejudice.

What has been going on within the universe of whites who rank high in racial resentment?

I posed this question to Michael Tesler, a political scientist at the University of California-Irvine who has focused much of his research on the role of race in politics, including as one of the authors of the book “Identity Crisis: The 2016 Election and the Battle for the Meaning of America,” which he wrote with Lynn Vavreck and John Sides, political scientists at U.C.L.A. and Vanderbilt.

Tesler replied that the share of the electorate made up of resentful white voters

has declined somewhat — as the country diversifies, whites have become more racially liberal. Their partisanship has also changed — particularly among low-educated whites who score high in racial resentment. Highly educated whites who score high in racial resentment have long been strongly Republican. This makes sense since they’ve long been aware of the fact that Republicans are the more racially conservative party.

Many less educated whites, in contrast,

were unaware of the parties’ differences on race before Obama’s presidency. Obama and Trump have helped simplify the politics of race by making it clear to even non-college voters who don’t pay much attention to politics where the two parties stand on race.

The result, according to Tesler:

Low-educated white voters in general, and low-educated voters who score high in racial resentment in particular, fled the Democratic Party.

Tesler wrote that while they were doing the research for “Identity Crisis,” he and his co-authors, using panel studies that tracked the same voters over time, found that measures of racial resentment, anti-Muslim attitudes and anti-immigrant sentiments “were all stronger predictors of vote choice in 2016 than in 2012.”

Brian Schaffner, a political scientist at Tufts and co-principal investigator at the Cooperative Congressional Election Study, elaborated on this point in an email:

I do not think Trump has made people more racist, but I do think that he has helped make explicit what was once implicit; namely his willingness to attack racial minority groups has led to a sharpening of the party divisions on attitudes related to race.

In a 2018 paper, “Understanding White Polarization in the 2016 Vote for President: The Sobering Role of Racism and Sexism,” Schaffner, Matthew C. MacWilliams and Tatishe Nteta, political scientists at the University of Massachusetts-Amherst, demonstrated that

while economic dissatisfaction was an important part of the story, racism and sexism were much more impactful in predicting support for Trump among white voters.

Schaffner and his colleagues continued:

Trump’s willingness to make explicitly racist and sexist appeals during the campaign, coupled with the presence of an African-American president and the first major party female nominee made racism and sexism a dividing line in the vote.

Lee Drutman, a senior fellow at the New America foundation and contributor to the Democracy Fund’s Voter Study Group, noted in an email that analysis of the voting of key subgroups is critical to understanding “the role of racial attitudes in the 2016 election.”

From this vantage point, Drutman argued, the critically important ballots were cast by vote-switchers from one party in 2012 to the opposite party in 2016:

By almost every account, Obama-to-Trump voters were pivotal in rust belt states that swung the Electoral College, and by almost every account, these voters had high racial resentment and anti-immigration attitudes.

In other words, Trump’s ability to convert and turn out these voters was the engine driving his election.