Among African-Americans, political solidarity surged in Democrats' favor after Republican presidential nominee Barry Goldwater opposed the Civil Rights Act of 1964. A similar trend occurred among Hispanics after 2004, as Republicans took a hard line against legal status for undocumented immigrants.
Lately, solidarity among whites has increased in Republicans' favor. As the U.S. Census projects America will become a majority minority nation within three decades, psychologists at UCLA in a 2015 study found fear among whites of losing their status as the "prototypical" ethnic group.
That fear, they wrote, represents "a novel, emerging source of resistance toward diversity in 21st Century America." Trump harnessed it better than anyone in last year's Republican field, as the first African-American president prepared to leave office.
In early 2016, political scientists from the University of Michigan and Vanderbilt examined data from the American National Election Study on voter attitudes toward candidates and different racial, ethnic and religious groups.
Their conclusion: Group solidarity among whites represented Trump's critical edge.
"Ethnocentrism among white Americans explains Trump's astonishing emergence," concluded Donald Kinder and Cindy Kam. In November, Trump defeated Clinton by 20 percentage points among whites.
Yet the concrete allocation of governing rewards will test the allegiance of two different groups of Trump supporters: financially squeezed blue-collar voters who welcome crackdowns on immigration, trade deals and crime; and affluent pro-business Republicans who seek cuts in taxes and regulations.
The Affordable Care Act that Trump pledges to repeal would eliminate insurance subsidies for many blue-collar workers who backed him, while cutting taxes for the wealthy. The tax cut plan he campaigned for would also disproportionately benefit high-earners.
In the name of working-class allies, Trump has scuttled the Trans-Pacific Partnership trade deal. That disappointed business supporters who have flourished in the globalized economy. His proposed travel ban, now stymied in federal court, triggered complaints from Silicon Valley and other multinational U.S. corporations.
In any event, solidarity within Trump's coalition could hold. An experiment by British psychologists, offering different groups of schoolboys choices of monetary rewards, found that individual teenagers chose options that benefited their groups, even if they received less themselves.
Members of groups, Vanderbilt's Kam explained, often prefer "the self-esteem they derive from where their in-group stands," regardless of material self-interest.
Yet research also shows that such solidarity weakens with higher levels of education. That means that upscale Trump supporters may be most susceptible to defection if policies adversely affect them, Kam speculated. The NBC/WSJ poll shows that, compared to Election Day, Trump's standing among college-educated whites has fallen slightly more than among non-college whites.