More famously, he helped drive the so-called birther movement, insisting that President Obama was not born in the United States and that investigators he had sent to Hawaii would expose “one of the greatest cons in the history of politics and beyond.”

Other times, he seems to be wondering aloud, as he did when he suggested the Clintons might have been involved in what he termed the “ very fishy ” 1993 suicide of former White House aide Vince Foster.

Sometimes, Donald Trump sounds as though he is just passing on information, as he did after Supreme Court Justice Antonin Scalia died. “They say they found a pillow on his face,” Trump told a radio interviewer , “which is a pretty unusual place to find a pillow.”


Trump’s affinity for conspiracy theories might seem the stuff of a few kooks and cranks living in their parents’ basement.

But far from being a marginal phenomenon, conspiracy theories have always been part of the American political landscape and are believed by more than 55 percent of the public — a group that cuts across race, gender, income, and political affiliation, according to researchers and polls.

The surprising breadth of conspiracy beliefs shows that while Trump’s rhetoric may repel a large segment of voters, it is also tapping a deep vein of thought among Americans who distrust elites and suspect that larger, darker forces are orchestrating domestic and world events.

“When I started studying conspiracy theories, I was stunned,” said Thomas J. Wood, a political scientist at Ohio State University. “I thought I was going to find them on the fringes of American attitudes, but they are a core way that Americans read about and explain political phenomena in response to uncertainty.”


What’s unusual, he said, is to have the presumptive nominee of one of the two major political parties using his stature to push such theories out of the realm of supermarket tabloids and e-mail chain lettters and into the political mainstream.

In prime-time debates, TV interviews, and on his Twitter account with 9.4 million followers, Trump has broadcast conspiracy theories that falsely suggest vaccines cause autism, global warming is a Chinese hoax, Obama sympathizes with ISIS, and Ted Cruz’s father might have been involved in the Kennedy assassination.

“In my estimation, what he’s doing is very scary,” said Joseph E. Uscinski, a political scientist at the University of Miami who noted that conspiracy theories are often espoused by despots. “He’s the head of the party and the nominee for president and he has a lot of power because of that, and conspiracy theories in the hands of powerful people generally lead to deleterious consequences.”

Supreme Court Justice Antonin Scalia. Josh Reynolds/Associated Press/File

Historians point out that conspiracy theories fueled the Anti-Masonic Movement in the 19th century, which alleged that Freemasons were secretly plotting to control the country; the Red Scare after World War II, which saw the creeping threat of Communism in every corner of society; and, more recently, dire warnings that the president’s health care law would lead to “death panels” that would cut off care to the elderly and disabled.

Populist leaders throughout history have also been drawn to theories about secret plots and powerful cabals because they promote “the sense that all elites are bad, that outsiders are bad, and the common people good,” said J. Eric Oliver, a University of Chicago political scientist. “Trump exemplifies this in spades.”


Trump’s most prominent embrace of a conspiracy theory was in 2011, when he was considering a White House run and raised his political profile by stoking the long-simmering rumor that Obama was not born in the country.

“I just say, very simply, ‘Why doesn’t he show his birth certificate?’ ” Trump asked on CNN in April 2011. “Why has he spent over $2 million in legal fees to keep this quiet and to keep this silent?”

Two weeks later, Obama released his long-form birth certificate, hoping to quash the claim. But a year later, Trump was unpersuaded, declaring on CNBC that “nothing has changed my mind” and that he still had “major questions” about whether Obama was eligible to serve as president.

Unlike conventional politicians who might worry about the blowback, Trump has also been a guest on the radio show of Alex Jones, a prominent conspiracy theorist who has suggested that the government was behind the Boston Marathon bombings and the Sept. 11, 2001, attacks .

A member loyal to the Islamic State in Iraq and the Levant (ISIL) waved an ISIL flag in 2014. REUTERS/Stringer

And Trump’s longtime friend and ally, Roger Stone, is a veteran Republican operative who has argued that Lyndon B. Johnson was behind the Kennedy assassination and that John F. Kennedy Jr. — who died in a plane crash — was murdered by the Clintons.


Jeff Hunt, director of the Centennial Institute, a think tank at Colorado Christian University, which will host Trump, Sarah Palin, and other conservatives at a summit this week, said he is “not a fan” of Trump’s conspiracy theories, but doesn’t pay much attention to them.

“I often hear them pop up, and you go, ‘Is he responding to something? Or is he floating it?’ ” said Hunt, who plans to vote for Trump in November. “I don’t see it as deliberately misleading but, if it is, it’s clearly something that should be condemned.”

When Trump entertains such theories, he often argues that he is merely relaying them, not endorsing them, as he did when he suggested in February that Cruz and Marco Rubio were not eligible to be president.

“Somebody said he’s not. And I retweeted it,” Trump said when questioned about the claim during an ABC interview. “I retweet things and we start dialogue and it’s very interesting.”

That approach makes it easier for Trump to evade responsibility for promoting falsehoods and misinformation, said Mark Fenster, a University of Florida law professor and author of “Conspiracy Theories: Secrecy and Power in American Culture.”

“He’s never saying, ‘I believe this,’ ” Fenster said. “He always puts it in terms of, ‘I don’t know; some people think so.’ And the great advantage of that is, there’s no follow-up. It’s a very strategic position that he takes.”


To ascertain just how prevalent such theories are, Wood and Oliver polled 1,935 Americans about six different common conspiracy theories in 2011, and found that 55 percent agreed with at least one.

The most widely held conspiracy theory, believed by 25 percent of respondents, asserted that the financial crisis was secretly orchestrated by a small group of Wall Street bankers to extend the power of the Federal Reserve and further their control of the world’s economy.

Almost as popular was the conspiracy theory, believed by 24 percent of Americans, that Obama was not born in the United States and does not have an authentic Hawaiian birth certificate.

And 19 percent of those polled said they believe the government planned the Sept. 11 attacks; that billionaire George Soros is behind a hidden plot to control the government, the media, and the world; and the invasion of Iraq was driven by oil companies and Jews in the United States and Israel.

US President Barack Obama made a statement on his birth certificate at the White House in Washington, DC, on April 27, 2011. JEWEL SAMAD/AFP/Getty Images

People are more likely to believe such theories when they feel threatened by war, terrorism, and economic upheaval, said Joseph M. Parent, a political scientist at the University of Miami and coauthor, with Uscinski, of “American Conspiracy Theories.”

“What they have in common is that they are emotional ice bags to recover from loss,” Parent said. “It can’t be your fault that something is going bad, somebody had to be conspiring, somebody had to do something dark and dangerous to take something away from you… and if you can just get back at them, your problems are solved.”

While conspiracy theories might tarnish Trump’s appeal in a general election, some of his supporters see them as less important than the candidate’s willingness to take a hard line against illegal immigrants and Muslims entering the country.

Trump’s outrageous claims about Foster’s death and Obama’s birth certificate “are just a lot of static,” said Mike Stopa, a physicist and Trump delegate from Massachusetts. “As long as the basic message and the policies he’s focusing on stay true, that’s what his supporters are there for.”

Michael Levenson can be reached at mlevenson@globe.com. Follow him on Twitter @mlevenson.