Some experts also think the current era exhibits something new – conspiracy charges that leave off the theory part. Harvard politics professor Nancy Rosenblum calls this “conspiracism.” Its force is often packed in a single word: “corrupt,” “rigged,” “treason.” In conspiracism, the fantastical claim comes first. The search for evidence happens later, if at all.

Today people also may see conspiracy theories as a means to impose a narrative on events they find inexplicable and threatening. After all, it’s a natural impulse to try to make sense out of things that seem random. In that context conspiracy theories can be a sort of pathway through seemingly dangerous times. Many Democrats still struggle to understand why President Donald Trump won in 2016. Many Republicans see the diversifying demographics of the U.S., and worry it threatens their vision of a predominantly white, Christian America – as well as their party’s future.

In America conspiratorial thinking took root with the arrival of the colonists. Every party that has ever taken part in the nation’s politics, at one time or another, has embraced allegations that networks of secret power are working toward some hidden purpose, according to Rachel Hope Cleves, a historian who studies conspiracy theories in public life.

Conspiracy theories have been intertwined with American politics since 1776 – perhaps earlier. But today they may be at the center of national public life more than ever in modern times.

Take President Donald Trump’s pressure on Ukraine. It was partly inspired by a discredited tale involving the Democratic National Committee, a cyber firm named CrowdStrike, and a server spirited to Ukraine, allegedly to hide the fact that Russia didn’t hack the 2016 U.S. election.

Prior to that citizen Donald Trump got his political start pushing the discredited “birther” theory that President Barack Obama was born in Kenya, and thus ineligible for office. “Growing up no one knew him,” Mr. Trump said falsely in a 2011 interview. “The whole thing is very strange.”

The left can think conspiratorially, too. Democratic Twitter “experts” who feverishly connect dots to prove that President Trump is Vladimir Putin’s paid agent can attract hundreds of thousands of followers. Earlier this month, Hillary Clinton said Democratic presidential candidate Tulsi Gabbard is being “groomed” by Republicans for a third party run in 2020. Ms. Clinton also called Representative Gabbard “a favorite of the Russians,” spawning a wave of social media conspiracy theories.

Why are conspiracy theories so prevalent now? One reason may be today’s us-vs-them, polarized political era. When the other side is deemed a villain, conspiracy theories are easier to accept. Maybe they don’t seem quite as outlandish as they otherwise might.

Today people also may see conspiracy theories as a means to impose a narrative on events they find inexplicable and threatening. After all, it’s a natural impulse to try to make sense out of things that seem random. In that context conspiracy theories can be a sort of pathway through seemingly dangerous times. Many Democrats still struggle to understand why President Trump won in 2016. Many Republicans see the diversifying demographics of the U.S., and worry it threatens their vision of a predominantly white, Christian America – as well as their party’s future.

Some experts also think the current era exhibits something new – conspiracy charges that leave off the theory part. Harvard politics professor Nancy Rosenblum calls this “conspiracism.” Its force is often packed in a single word: “corrupt,” “rigged,” “treason.” In conspiracism, the fantastical claim comes first. The search for evidence happens later, if at all.

“It is the property of the right today, but if it is effective ... it is quite possible people on other parts of the spectrum would adopt it,” says Dr. Rosenblum.

Conspiracies arrived with the colonists

In America conspiratorial thinking took root with the arrival of the colonists. Every party that has ever taken part in the nation’s politics, at time or another, has embraced allegations that networks of secret power are working towards some hidden purpose, according to Rachel Hope Cleves, a historian at the University of Victoria, British Columbia, who studies conspiracy theories in public life.

Early in the republic the populist Jeffersonian faction of leaders warned that establishment Federalists, such as Alexander Hamilton, were plotting to make America a monarchy. In the election of 1800 some conservatives charged in return that Thomas Jefferson and his followers were in league with the Illuminati, a group of European elites plotting to overthrow governments, seize private property, and destroy Christianity.

Prior to the Civil War many Northerners feared that a shadowy Southern “slave power” controlled the nation’s government via murder and blackmail. A popular 1864 book, “The Adder’s Den,” was a conspiracy manifesto that charged the “slave power” had tried to assassinate President James Buchanan by poisoning all the sugar cubes in Washington’s National Hotel.

Conspiracy theories targeting Freemasons were rife in America’s early days. In the 20th century some conservatives charged that fluoridation of the U.S. water supply was a communist conspiracy to weaken the nation. Sen. Joseph McCarthy’s Red Scare conspiracy claimed the Soviet Union had infiltrated virtually every department of government. The assassination of John F. Kennedy has spawned dozens of conspiracy theories, which weave in Cuba, the Mafia, the C.I.A., and Lyndon Johnson as suspects.

Conspiracy theories reflect a mode of thinking, say those who study them. Some are true. Some have a grain of truth. Many are false.

In the U.S. this mode of thought is rooted in the 18th century republican belief that power is ambitious and expansive and liberty is always on the defensive, according to Dr. Cleves. This was labeled the “paranoid style in American politics” by historian Richard Hofstadter in a famous essay in 1964.

People and parties who feel powerless are attracted to conspiracy theories, say experts. They can provide comforting explanations for their predicament. But sometimes they are attractive to the powerful as well – if the powerful are uneasy in their position.

The Federalists, for instance, held the presidency when some of them pushed the fantastical Illuminati charges against their foes. They could see that their hierarchical vision of the U.S. republic would likely be supplanted by the Jeffersonians’ more egalitarian vision, says Dr. Cleves. A sea change was coming and they felt threatened.

That could be comparable to the position today of the Republican Party, she says. The electorate is moving quickly toward becoming majority-minority. The whites who predominate in the GOP could feel threatened.

“Conspiratorial thinking might be attractive to [today’s GOP-controlled] executive branch because it senses a vulnerability of future political structure. It’s under threat from shifting demographics and shifting ideals of who belongs in power,” Dr. Cleves says.

Intuitionists vs. rationalists

In America, conspiracy theories aren’t just for people who wear proverbial tin foil hats. A substantial number of voters subscribe to surprising beliefs about politics and the nation at large.

University of Chicago political science professor Eric Oliver says this came as a big shock to him when he began studying conspiracy theories years ago. He says if you take a survey listing five or six popular conspiracy theories, such as the false charge that NASA faked the moon landing, or that Lee Harvey Oswald was a CIA plant, half of all respondents will mark at least one as “true.”

NASA/File This close-up view shows an Apollo 11 astronaut's footprint in the lunar soil, photographed with a 70mm camera during the extravehicular activity on the moon in July 1969.

The one that typically gets the most support, he says, is the untrue charge that the F.D.A. is withholding naturally derived cures for cancer due to pressure from big food and drug firms.

When President Obama was in office, 23% to 25% of Americans said they believed in the “birther” fraud, Dr. Oliver points out. About 19% today claim to believe the “truther” lie that 9/11 was an inside job.

People with supernatural views – who believe in ghosts and extra-sensory perception – are the most prone to ascribe truth to conspiracy theories, according to Dr. Oliver. He divides the electorate into two groups: intuitionists, who draw on their own feelings and gut instincts to make sense of the world; and rationalists, who put more stock in studies and facts.

“Right now we are seeing the public polarizing in these two world views,” Dr. Oliver says.

The right wing of the political spectrum is becoming more intuitionist, he says. There are still leftists who ascribe to conspiracy theories – the anti-vaccine movement, for instance, is concentrated in left-leaning areas. But the movement of evangelical Christians into the Republican Party, and the rise and influence of President Trump, have greatly increased the GOP’s susceptibility to conspiratorial thinking, according to Dr. Oliver.

“President Trump is the embodiment of an intuitionist thinker,” he says.

President Trump’s embrace of the false narrative of Obama “birtherism” is perhaps his most well-known embrace of a political conspiracy theory. He was the most prominent person to push the discredited notion that President Obama was born in Kenya rather than Hawaii, and it helped raise his political profile in the years leading up to his jump into presidential politics.

In September 2016, as the election approached, Mr. Trump acknowledged that “President Barack Obama was born in the United States, period.” But this statement was brief and came at the end of a press appearance at which he extolled the virtues of his new Washington hotel.

The conspiracy theory that the president appears to be embracing most at the moment involves the cyber defense firm CrowdStrike and Ukraine. That is what he was referring do in his famous phone call with Ukrainian President Volodymyr Zelenskiy when he said “The server, they say Ukraine has it.”

The CrowdStrike theory is almost baroque in its complications. Simplified, it holds that the hacking of DNC emails prior to the 2016 vote was not carried out by Russia, as U.S. intelligence holds, but Ukraine – and it was designed to help Mrs. Clinton, by pinning the deed on the Kremlin. Proof of this allegedly lies in an American computer server that has been hidden in Ukraine to keep it safe.

CrowdStrike, the private firm that first detected the DNC intrusion, is complicit, goes the theory. It also supposedly is partly owned by a Ukrainian oligarch (it isn’t, actually) which supposedly explains the hidden-server-in-Ukraine connection.

The list of things that would have to be true for the CrowdStrike theory to make sense is a long one. Among other things, hundreds of people, including Deep State bureaucrats, the heads of U.S. intelligence and the Justice Department, and numerous foreign officials, would have to be in on the scam.

Mr. Trump’s first homeland security adviser, Thomas Bossart, warned the president this theory was “completely debunked.” Yet President Trump in July appeared to be very interested in a Ukrainian investigation of the subject. Whether he withheld military aid to force such a probe, as well as a look at business dealings in Ukraine by Hunter Biden, is at the center of the House impeachment inquiry.

Conspiracy without theory

Conspiracy theories are an old political trope. But conspiracy without theory is something new, and suddenly prevalent, according to Dr. Rosenblum of Harvard University, co-author of the new book “A Lot of People are Saying: The New Conspiracism and the Assault on Democracy.”

“We’ve never had a president with a conspiracy-minded mindset like Trump’s,” she says.

She labels the new approach “conspiracism.” It operates with bare assertion, she says: the election is “rigged,” the press is “corrupt,” Democrats are engaging in “treason.” In their breadth these charges are almost impossible to prove or disprove. They get power from constant repetition, Dr. Rosenblum says.

Conspiracism is disorienting because it is an assault on our sense of reality, and on the knowledge-based institutions that might challenge conspiratorial thinking. As employed by the president, it does not seem to have an ideological or political purpose, beyond keeping him in office, she adds.

Social media is fertile ground for conspiracism to spread. Currently it is used by the right, but the left is not immune from conspiracy thinking in general, so there is no reason to think a Democrat with a Trumpian approach isn’t possible, Dr. Rosenblum says.

Repeating truth in the face of conspiracy is one way to fight it, she says. But it is elected officials, people with a partisan connection to their constituents, who have a particular responsibility to stand up to such attacks.

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“There are all kinds of institutional ways to resist,” she says.

Note: An earlier version of this story stated that Hillary Clinton said Russia was “grooming” a Democrat aligned with Kremlin interests to run as a third-party candidate in the 2020 election. Ms. Clinton actually said it was the Republicans who were grooming the candidate, whom she also called “a favorite of the Russians.”