It’s possible to gaze back almost wistfully on all the moments in recent American politics that seemed, at the time, to constitute Peak Crazy, but look from today’s vantage point like false summits. Consider, for instance, the remarks Barack Obama gave at the 2011 White House Correspondents Dinner, three days after releasing the long-form birth certificate Donald Trump had spent weeks demanding. “Now, I know that he’s taken some flak lately,” Obama said, with Trump in the audience, “but no one is happier, no one is prouder to put this birth-certificate matter to rest than the Donald. And that’s because he can finally get back to focusing on issues that matter — like, did we fake the moon landing? What really happened in Roswell? And where are Biggie and Tupac?”

But of course, here we are in 2016, with the butt of Obama’s joke polling just single digits behind Hillary Clinton in the presidential race. Trump did finally concede in September that Obama was indeed born in the United States, perhaps just to make room in his stump speeches for even more extravagant conspiracy theories. He has insisted repeatedly that Obama, or Clinton, or both, personally founded the Islamic State. He called California’s five-year drought a myth propagated by environmentalists and suggested that Ted Cruz’s father might have been involved in the Kennedy assassination — something 7 percent of voters with a favorable opinion of Trump told one pollster they believed. In December he gave an interview to Alex Jones, a conspiracy theorist whose talk show and websites reach an audience of millions. “Your reputation is amazing,” he told Jones. “I will not let you down.”

This was enough to earn Jones a line in the speech Clinton gave in Reno, Nev., in August, excoriating Trump’s “dis­credited conspiracy theories with racist undertones.” Trump’s worldview, Clinton said, is “what happens when you listen to the radio host Alex Jones, who claims that 9/11 and the Oklahoma City bombings were inside jobs.” This year, the woman who once bemoaned a “vast right-wing conspiracy” arrayed against her and her husband is left trying to demarcate the real world from a toxic, imagined one.

Clinton’s denunciation of “the paranoid fringe in our politics” was a clear nod to “The Paranoid Style in American Politics,” the 1964 essay by the liberal historian Richard Hofstadter. Hofstadter described a miasma of “conspiratorial fantasy” that had coursed through American public life since the earliest days of the nation, a litany of feverish plots involving the Bavarian Illuminati, the Freemasons and Jesuit priests. These manias had always ranged freely across the political spectrum, but in the postwar period, Hofstadter wrote, “we have seen angry minds at work mainly among extreme right-wingers” — from Joseph McCarthy’s congressional witch hunts to the John Birch Society to Barry Goldwater’s Sun Belt insurrection.