When President Biden announced he was suspending his campaign for president, Donald Trump and some ofhis supporters stitched together a series of conspiracy theories about Biden’s health, his motives for dropping out, and even whether he was still alive.

Those false assertions built on years of reality-bending messages from Trump and others on the right that have helped polarize the electorate and shaken Americans’ belief in a shared set of facts. While political conspiracy theories have long been a feature of American life, today’s diffuse army of conspiracists areespecially able to sow doubt, experts say, and are poised to undermine faith in the coming election less than four years after such false conspiracy theories fomented a violent attack on the Capitol.

Hours after Biden said on Sunday that he was dropping out of the race,which came days after he received his covid diagnosis, Trump set the tone in a social media post. “Does anybody really believe that Crooked Joe had Covid? No,” Trump wrote, challenging the announcement from Biden’s own physician describing his symptoms.

In the same post, Trump claimed, without evidence, that Biden “had wanted to get out” of the presidential race since the night of the debate that sparked concerns nationally over the ability of Biden, 81, to serve a second term in the White House. (Biden acceded to calls that he resign from the race only after weeks of pressure from members of his own party.)

“What happened to Joe Biden,” Trump adviser Chris LaCivita posted on X on Monday. “Where is he? We haven’t seen him since July 17th … around the time he ‘got covid’.”

“When was the last time anyone saw Joe Biden?” asked Charlie Kirk, the founder and CEO of Talking Points USA, a youth-focused conservative advocacy group, who spoke at last week’s Republican National Convention in Milwaukee.

Trump and his allies provided no evidence for their conjecture, and the false assertions built up to the allegation advanced by some on the far right that Vice President Harris’s elevation to succeed Biden as the Democratic presidential candidate amounted to an illegal government takeover.

“President Trump is merely asking relevant questions — something the media should do after they were exposed for helping Biden cover up his cognitive decline for the past four years,” said Steven Cheung, communications director for Trump’s campaign.

Even before Biden announced he was suspending his campaign, Trump supporters predicted his covid diagnosis would keep him from running. Joey Mannarino, a self-described conservative populist with half a million followers on X, posted that Biden would get long covid, a condition he dismissed as fake, and quit the race. “Just wait,” he wrote.

The day Biden released a letter announcing his decision to leave the race, Laura Loomer, a right-wing anti-Muslim activist with a history of publishing falsehoods, wrote to her more than a million followers on X that she had heard from an unnamed source that Biden had not written the letter. Feminist author turned conspiracist Naomi Wolf reposted Loomer’s message. Another account, Slave to Christ, posted an image of Biden’s signature at the base of the letter, claiming that it did not match previous examples of his penmanship.

Wolf in turn reposted the message, which spurred hedge fund manager Bill Ackman to chime in, questioning the authenticity of Biden’s signature to his 1.3 million X followers. The conspiracy theories persisted even after Biden called in to Harris’s televised appearance Monday at campaign headquarters in Wilmington, Del., to rally staffers’ support for the vice president — his first public remarks since dropping out of the race.

Kirk on Monday posted on X that he had had “a weird lead” on a story from a source close to the police departmentin Las Vegas, where Biden visited last week,that contradicted the “official story” that Biden had contracted covid. Kirk’s source,he said, indicated that the U.S. Secret Service had told the local police that an “emergency situation” involving Biden required a “medevac” to transport him to Johns Hopkins Medical Center in Baltimore. “Apparently the rumor mill in the police department was that Joe Biden was dying or possibly already dead,” Kirk wrote to his more than 3 million followers. Kirk did not specify why his unnamed source asserted that Biden would travel to Johns Hopkins. He arrived later that evening at Dover Air Force Base in Delaware.

Kirk wrote that he had dismissed the tip at the time, but given that Biden had not been seen in public for several days, Kirk wrote that he was curious “if there is more to the official story than what they’re telling us.”

Tucker Carlson read Kirk’s dispatch on his online show hosted on X, and several conservative outlets picked up the report.

When asked about the reports, the Las Vegas Metropolitan Police Department provided a statement that it “was notified that President Joe Biden was sick on July 17th during his visit to Las Vegas” but were not informed of the nature of his illness. “As a precaution, LVMPD proactively began to shut down roads” leading to the local hospital, before the Secret Service told the police that Biden was heading directly to the airport.

“I think it’s clear that there was an undisclosed medical event on July 17th,” Andrew Kolvet, a spokesman for Kirk, texted, citing the planned road closures and additional anonymous police sources from another journalist, but no medical evidence. After Kirk received a tip, he shared his information with his audience to try to get more detail, Kolvet added. “Joe Biden either recovered quickly, or they’re not telling us the entire story. It’s also very curious that he dropped out of the race only four days later. We still have a lot of questions, but we followed a lead and have confirmed much of the original story.”

Heading into the last months of the 2024 presidential campaign, which has been upended by extraordinary events including an assassination attempt on Trump and Biden’s decision to step away from the race, researchers are girding for conspiracy theories to take root with every news event.

“It’s a common tactic to take a shred of information that’s in the news and find a way to align it with an existing conspiratorial worldview, which in this case is that the Deep State and the current administration are out to get Trump,” said Megan Squire, deputy director for data analytics at the Southern Poverty Law Center. She explained that conspiracy theories about Biden’s ill health serve to demonstrate that the federal government is dishonestly propping up a Trump adversary in order to undermine the former president. “Most of these theories are designed to position Trump as the hero of the story,” she added.

Some figures on the right attempted to tie the attempted assassination to Biden’s stepping out of the race.

“Biden didn’t drop out of the race until after the attempts to imprison and assassinate Donald Trump failed. Do you think that is coincidence?” Sean Davis, co-founder and CEO of the conservative Federalist site, said in a post shared on X.

“All political conspiracy theories seem to eventually converge,” said Melissa Ryan, digital strategist at Card Strategies, where she specializes in studying online hate and conspiracy theories. She added that the close proximity in time between Trump’s shooting and Biden’s withdrawal made it easier for the conspiracy-minded to link the two events.

But investigators have found no evidence connecting Biden or the Democratic Party to the gunman who attempted to kill Trump, nor have they identified a particular ideology or motive driving the shooter, identified asThomas Matthew Crooks, a 20-year-old killed at the scene. Crooks had searched online days earlier for information about the assassination of John F. Kennedy, and he used a rifle with a collapsible stock that may have made it easier for him to disguise the weapon before climbing onto a roof, FBI Director Christopher A. Wray told the House Judiciary Committee on Wednesday.

The current moment has brought conspiratorial thinking, which emerged on the fringes of politics during Trump’s first term, into the center of a major-party platform, experts said.

“The issue isn’t so much the theories themselves,” said Joseph E. Uscinski, a political scientist at the University of Miami who specializes in conspiracy theories. “The issue is that you have Donald Trump and his allies in the conservative media and the government using a lot of conspiracy theories, and that’s where it becomes a major problem, because they have political elites like presidential candidates and senators and representatives and cable news channels and internet personalities with big audiences … pushing this stuff to audiences who trust them. [And] that’s really bad. And that’s where we need to be focused.”

Before Harris addressed the nation Tuesday from a suburb of Milwaukee in her first rally, Worldnet Daily, the far-right conspiracy theory publication that was a leading voice falsely claiming that former president Barack Obama was not born in the U.S., recirculated online posts about Biden stepping away from the race and asked, “Did we just witness a coup?”

The inability to agree on a set of facts leads to the perception of an information vacuum, said Squire, such that even with more information, the vacuum never fills up. “When you have a lack of information,” she said, “the world we’ve built is going to fill that with garbage.”

Jeremy Merrill contributed research.