After losing an ugly congressional race last year, Denver Riggleman is leading a charge against the conspiracy-mongering coursing through his party. He doesn’t have many allies.AFTON, Va. — Denver Riggleman stood virtually alone.It was Oct. 2, on the floor of the House of Representatives, and he rose as one of only two Republicans in the chamber to speak in favor of a resolution denouncing QAnon. Mr. Riggleman, a freshman congressman from Virginia, had his own personal experiences with fringe ideas, both as a target of them and as a curious observer of the power they hold over true believers. He saw a dangerous movement becoming more intertwined with his party, and worried that it was only growing thanks to words of encouragement from President Donald J. Trump.“Will we stand up and condemn a dangerous, dehumanizing and convoluted conspiracy theory that the F.B.I. has assessed with high confidence is very likely to motivate some domestic extremists?” asked Mr. Riggleman, a former Air Force intelligence officer. “We should not be playing with fire.”Six months later, conspiracy theories like QAnon remain a threat that most Republicans would rather ignore than confront, and Mr. Riggleman is out of office. But he is ever more determined to try to expose disinformation from the far right that is swaying legions in the Republican base to believe in a false reality.Mr. Riggleman is a living example of the political price of falling out of lock step with the hard right. He lost a G.O.P. primary race last June after he officiated at the wedding of a gay couple. And once he started calling out QAnon, whose followers believe that a satanic network of child molesters runs the Democratic Party, he received death threats and was attacked as a traitor, including by members of his own family.The undoing of Mr. Riggleman — and now his unlikely crusade — is revealing about a dimension of conservative politics today. The fight against radicalism within the G.O.P. is a deeply lonely one, waged mostly by Republicans like him who are no longer in office, and by the small handful of elected officials who have decided that they are willing to speak up even if it means that they, too, could be headed for an early retirement.“I’ve been telling people: ‘You don’t understand. This is getting worse, not better,’” Mr. Riggleman said, sitting on a stool at his family bar one recent afternoon. “People are angry. And they’re angry at the truth tellers.”Mr. Riggleman, 51, is now back home in the foothills of the Blue Ridge Mountains, where he and his wife run the bar and a distillery. And for his next move in a career that has included jobs at the National Security Agency and founding a military contracting business, he is working with a group of other experts to shine a light on what he calls the “social disease” of disinformation.His experience with the issues and emotions at work is both professional and personal. He was so intrigued by false belief systems that he self-published a book about the myth of Bigfoot and the people who are unshakably devoted to it.Mr. Riggleman is working with a group of other experts to shine a light on what he calls the “social disease” of disinformation.Matt Eich for The New York TimesMr. Riggleman, who first ran and won in 2018 after the Republican incumbent in his district retired, joined the arch-conservative Freedom Caucus and was endorsed by Mr. Trump. Now he says it “gives me shivers” to be called a Republican. He hopes to show that there is still a way to beat back the lies and false beliefs that have spread from the fringe to the mainstream. It is a heavy lift, and one that depends on overcoming two strong impulses: politicians’ fear of losing elections and people’s reluctance to accept that they were taken in by a lie.Mr. Riggleman summarized his conversations with the 70 percent of House Republicans he said were privately appalled at the former president’s conduct but wouldn’t dare speak out.“‘We couldn’t do that in our district. We would lose,’” he said. “That’s it. It’s that simple.”Stocky, fast-talking and inexhaustibly curious, the former congressman is now working for a group of prominent experts and academics at the Network Contagion Research Institute, which studies the spread of disinformation in American politics and how to thwart it. The group has undertaken several extensive investigations into how extremists have used propaganda and faked information to sow division over some of the most contentious issues of the day, like the coronavirus pandemic and police violence.Their reports have also given lawmakers a better understanding of the QAnon belief system and other radical ideologies that helped fuel the riot at the Capitol on Jan. 6.Mr. Riggleman said he had written one report about the involvement of far-right militants and white supremacist groups in the attack specifically at the request of a Republican member who needed help convincing colleagues that far-left groups were not the culprits.Getting lawmakers to see radical movements like QAnon as a threat has been difficult. Joel Finkelstein, the director of the Network Contagion Research Institute, said that in June, when the group tried to sound the alarm on QAnon to members of Congress, Mr. Riggleman was the only one who responded with a sense of urgency and agreed to help.“We were screaming it from the rooftops,” Mr. Finkelstein said. “We said: ‘This is going to be a problem. They’re growing increasingly militant in their conspiracies.’” When the institute’s members spoke to Mr. Riggleman, he said, “We showed him our data and he said, ‘Holy moly.’”Far from a theoretical or overblown concern, disinformation and its role in perpetuating false beliefs about Mr. Trump’s election loss and its aftermath are problems that some Republicans believe could cripple their party if left ignored.In a sign of how widespread these conspiracy theories are, a recent poll from Suffolk University and USA Today found that 58 percent of Trump voters wrongly believed the storming of the Capitol was mostly inspired by far-left radicals associated with antifa and involved only a few Trump supporters.“There was a troika of us who said, ‘This is going to a bad place,’” said Paul Mitchell, who represented Michigan in the House for two terms before retiring early this year in frustration. He said he had watched as members dismissed Mr. Riggleman, despite his experience in intelligence. “There weren’t many people who gave a damn what your expertise was,” Mr. Mitchell said. “It was inconsequential compared to the talking points.”Bob Good defeated Mr. Riggleman in a state Republican Party convention in June.Amy Friedenberger/The Roanoke Times, via Associated PressMr. Riggleman’s loss last summer in a closely held party convention allowed him to be more outspoken. The winner, Representative Bob Good, is a former associate athletic director at Liberty University who took issue with Mr. Riggleman’s officiation at the gay wedding and called him “out of step” with the party’s base.And as Mr. Riggleman kept it up and spoke out more aggressively against Mr. Trump after the election, his fight got lonelier.“I had a colleague of mine pat me on the shoulder and say: ‘Denver, you’re just too paranoid. You’re killing yourself for the rest of your life politically by going after the big man like this,’” Mr. Riggleman recalled.When he returned to Virginia for good in January, he said he sometimes felt just as isolated. Family members, former constituents and patrons at the distillery insisted that the election had been stolen from Mr. Trump. And they couldn’t be talked out of it, no matter how hard he tried.He recalled a recent conversation with one couple he is friends with that he said was especially exasperating.“I go over stats,” he said. “I go over figures. I go over the 50 states, how that actually works. How machines that aren’t connected are very hard to hack. How you’d have to pay off hundreds of thousands of people to do this.”“Did not convince them,” he added.Other friends of his, some of whom are also members of the growing group of former Republican lawmakers now publicly criticizing Mr. Trump, said that many conservative politicians saw no incentive in trying to dispel disinformation even when they know it’s false.“What some of these guys have told me privately is it’s still kind of self-preservation,” said Joe Walsh, a former congressman from Illinois who ran a short-lived primary campaign against Mr. Trump last year. “‘I want to hang onto the gig. And this is a fever, it will break.’”That is mistaken, Mr. Walsh said, because he sees no breaking the spell Mr. Trump has over Republican voters anytime soon. “It’s done, and it was done a few years ago,” he said.Mr. Riggleman, who is contemplating a run for governor in Virginia and is writing a book about his experience with the dark side of Republican politics, sees a way forward in his experience with Bigfoot. The sasquatch was how many people first learned about him as a politician, after an opponent accused him of harboring a fascination with “Bigfoot erotica,” in 2018.“I do not dabble in monster porn,” he retorts in his book, “Bigfoot … It’s Complicated,” which he based in part on a trip he took in 2004 on a Bigfoot expedition.Mr. Riggleman paid $2,000 to go on a Bigfoot expedition with his wife in 2004.Matt Eich for The New York TimesThe book is full of passages that, if pulled out and scrubbed of references to the mythical creature, could be describing politics in 2021.Mr. Riggleman quotes one true believer explaining why he is absolutely convinced Bigfoot is real, even though he has never seen it. In an answer that could have come straight from the lips of someone defending the myth that Mr. Trump actually won the 2020 election, the man says matter-of-factly: “Evidence is overwhelming. Check out the internet. All kinds of sightings and facts.”At another point, Mr. Riggleman describes a conversation he had with someone who asked if he really thought that all the people claiming to have seen Bigfoot over the years were liars. “I don’t think that,” Mr. Riggleman responds. “I do believe that people see what they want to see.”He did find one way to crack the Bigfoot false belief system: telling true believers that they were being ripped off to the tune of hundreds or thousands of dollars to go on expeditions where they would never actually see the creature.“They got very angry,” he said. But eventually, some started to come around. More