Ever since the rise of the tea party, a populist, libertarian-tinged movement that has morphed into the Trump groundswell, the most controversial question about it has been whether its members are animated, at least in part, by racism.

A new paper from three academics provides some of the most robust evidence to answer it. Put simply, the researchers found that a fear of the decline of white standing in the United States lies behind at least some degree of support for the tea party.

The researchers, led by Stanford University's Robb Willer, used five different methods to get at the same question. They found the results surprisingly strong, and surprisingly consistent.

“It’s such a controversial finding and theory that we were testing that we really wanted to be sure we had it nailed down before we wrote it up," Willer told The Huffington Post.

The findings appeared Wednesday on the site Social Science Research Network.

In one part of the study, researchers showed participants a picture of President Barack Obama. Some of the participants were shown a photo of Obama in which his skin had been artificially lightened; others saw a version of Obama whose skin had been artificially darkened. White people who saw the darker Obama were more likely to express support for the tea party.

For another part of the study, researchers asked participants to read some information about U.S. racial demographics and then answer a few questions. Some participants were given a report emphasizing that "whites remain the largest ethnic group in the U.S.," according to the study. Others were given a report emphasizing that "the white majority in the U.S. is steadily declining, with minorities expected to surpass whites in numbers by 2042." White people who read the second report were more likely to express support for the tea party.

For still another part of the study, participants were asked to read the report that emphasized the declining white population in the U.S. Then, they were shown a list of positions and beliefs associated with the tea party, and were asked about their level of support for the movement. Some participants were shown a list of tea party priorities that were classically libertarian, like reducing government regulation of businesses. Others were shown a list of "positions which past research finds are often associated with racial resentment," like "stricter policies against illegal immigration." Once again, white people who were shown the more explicitly race-based platform expressed a higher degree of support for the tea party.

To some critics of the tea party, this probably comes as no surprise. But many tea partiers have long insisted that race plays no role in their anger at the government -- just as, more recently, a lot of Trump fans say that race has nothing to do with their support of the real estate mogul.

Some observers have suggested that many white Americans find the tea party appealing because of things they notice in their daily lives -- little reminders that America is growing more diverse, not less, and that white people don't have the monopoly on economic prosperity and social capital that they used to. They look around and see, or think they see, more Muslims in their neighborhoods, or more immigrants taking jobs.

But Willer and the other researchers argue that it might not be these micro-level experiences that drive some white Americans to the tea party. Rather, they say, there's something larger going on -- an almost primal retreat into tribalism, a reaction to macro trends that many tea partiers may not even register on a conscious level.

“While our research most directly tested whether racial status threat shapes levels of Tea Party support, our results also speak indirectly to the historical roots of popular support for the movement,” the report says. “The Tea Party emerged during a period when white Americans’ political power was threatened by the election of Barack Obama, their majority status was threatened by a rising minority population that received wide media coverage, and the Great Recession increased their economic insecurity, a factor previously shown to catalyze racial threats.”

Trump, a birther who claims that African-Americans commit most violent crimes, has promised to fix this. He says he will (temporarily!) ban Muslims from entering the U.S. and crack down on Mexicans -- especially the “criminals” and “rapists” -- trying to get into the country.

Trump’s tirades, and his vows to "make America great again," imply that he can bring back a time when white men dominated politics and were the sole face of economic prosperity. "I love the old days," Trump said at a February rally where a protester was escorted out. "You know what they used to do to guys like that when they were in a place like this? They'd be carried out on a stretcher, folks."

It should be noted that Wednesday's study is about the tea party. It doesn't explicitly link racial anxiety with Donald Trump -- in fact, the word "Trump" never appears once in the paper. But the anger at Washington that Trump often ascribes to his supporters is a signature aspect of the tea party movement. And Trump's rise from political outsider to presumptive Republican nominee for president mirrors the success of the many tea party candidates who've been elected to Congress after running similar campaigns. It's not hard to draw a line from the early days of the tea party to Trump's ascent this election cycle. In a CNN poll in February, 56 percent of tea party voters said they supported Trump.

“It’s less surprising if you think about the tea party as a sort of historical bridge in [Trump’s] candidacy,” Willer said. “The tea party platform is not, for the most part, explicitly prejudiced. Our research shows that a lot of support for it derives from white Americans' feelings of threat and resentment. And now we find that Trump can push things a little further.”

Trump is fond of demonizing China as an economic powerhouse and suggesting that it's accruing prosperity that should be America's by right. Scapegoating people of color is an especially potent strategy during times of economic struggle, Willer noted.

“Plenty of other research shows that racial threat effects tend to be amplified when there’s an economic downturn," he said. "There's a sense that the dominant racial group has a shrinking portion of a pie that is itself shrinking.”

This racial resentment could explain the violence at Trump rallies as well. At least 25 incidents of physical violence occurred at Trump rallies between Feb. 29 and April 6, a HuffPost survey of police departments and news reports found.

“When we fit this lens of thinking about Trump supporters as driven in part by a feeling of declining racial status in America, it clarifies some of the violence we see,” Willer said.

“The feeling that a basis of personal and group esteem that one got very used to is now under threat -- that feeling of threat is a very strong, strong motivation,” Willer said. “It can provoke a lot of anger and hostility to feel like you had something that made you and your group valuable and that thing is being taken from you.”

The research suggests that the racialized response isn't occurring on the surface level, but rather that it comes from deep down. And it's not necessarily rooted in an active dislike or hatred of people of color, the thinking goes. Rather, it's a knee-jerk defense of the central role of white people in American society.

In other words, many people with latent feelings of racial anxiety and racial resentment aren't anti-black or anti-brown so much as they're pro-white. White nationalists, who have flocked to Trump's campaign, appear to understand that intuitively.

“With Trump, white supremacists understand that he’s not exactly a white nationalist, like them, but they applaud his hard-right positions on matters that are important to them,” said Mark Potok, senior fellow at the Southern Poverty Law Center. “From their point of view, it’s almost better that he’s not a full-on white nationalist, because now he has a better chance at winning a major office.”

Editor’s note: Donald Trump regularly incites political violence and is a serial liar, rampant xenophobe, racist, misogynist and birther who has repeatedly pledged to ban all Muslims — 1.6 billion members of an entire religion — from entering the U.S.