Looking back from that perspective, we can now see a history that is indeed unsettling — but also unsettlingly familiar. Consider, for example, an essay published in 1926 by Hiram Evans, the imperial wizard of the Ku Klux Klan, in the exceedingly mainstream North American Review. His subject was the decline of “Americanism.” Evans claimed to speak for an abused white majority, “the so-called Nordic race,” which, “with all its faults, has given the world almost the whole of modern civilization.” Evans, a former dentist, proposed that his was “a movement of plain people,” and acknowledged that this “lays us open to the charge of being hicks and ‘rubes’ and ‘drivers of secondhand Fords.’ ” But over the course of the last generation, he wrote, these good people “have found themselves increasingly uncomfortable, and finally deeply distressed,” watching a “moral breakdown” that was destroying a once-great nation. First, there was “confusion in thought and opinion, a groping and hesitancy about national affairs and private life alike, in sharp contrast to the clear, straightforward purposes of our earlier years.” Next, they found “the control of much of our industry and commerce taken over by strangers, who stacked the cards of success and prosperity against us,” and ultimately these strangers “came to dominate our government.” The only thing that would make America great again, as it were, was “a return of power into the hands of everyday, not highly cultured, not overly intellectualized, but entirely unspoiled and not de-Americanized average citizens of old stock.”

This “Second Klan” (the first was formed during Reconstruction) scrambles our pre-Trump sense of what right-wing ideology does and does not comprise. (Its doctrines, for example, included support for public education, to weaken Catholic parochial schools.) The Klan also put the predations of the international banking class at the center of its rhetoric. Its worldview resembles, in fact, the right-wing politics of contemporary Europe — a tradition, heretofore judged foreign to American politics, called “herrenvolk republicanism,” that reserved social democracy solely for the white majority. By reaching back to the reactionary traditions of the 1920s, we might better understand the alliance between the “alt-right” figures that emerged as fervent Trump supporters during last year’s election and the ascendant far-right nativist political parties in Europe.

None of this history is hidden. Indeed, in the 1990s, a rich scholarly literature emerged on the 1920s Klan and its extraordinary, and decidedly national, influence. (One hotbed of Klan activity, for example, was Anaheim, Calif. McGirr’s “Suburban Warriors” mentions this but doesn’t discuss it; neither did I in my own account of Orange County conservatism in “Before the Storm.” Again, it just didn’t seem relevant to the subject of the modern conservative movement.) The general belief among historians, however, was that the Klan’s national influence faded in the years after 1925, when Indiana’s grand dragon, D.C. Stephenson, who served as the de facto political boss for the entire state, was convicted of murdering a young woman.

But the Klan remained relevant far beyond the South. In 1936 a group called the Black Legion, active in the industrial Midwest, burst into public consciousness after members assassinated a Works Progress Administration official in Detroit. The group, which considered itself a Klan enforcement arm, dominated the news that year. The F.B.I. estimated its membership at 135,000, including a large number of public officials, possibly including Detroit’s police chief. The Associated Press reported in 1936 that the group was suspected of assassinating as many as 50 people. In 1937, Humphrey Bogart starred in a film about it. In an informal survey, however, I found that many leading historians of the right — including one who wrote an important book covering the 1930s — hadn’t heard of the Black Legion.

Stephen H. Norwood, one of the few historians who did study the Black Legion, also mined another rich seam of neglected history in which far-right vigilantism and outright fascism routinely infiltrated the mainstream of American life. The story begins with Father Charles Coughlin, the Detroit-based “radio priest” who at his peak reached as many as 30 million weekly listeners. In 1938, Coughlin’s magazine, Social Justice, began reprinting “Protocols of the Learned Elders of Zion,” a forged tract about a global Jewish conspiracy first popularized in the United States by Henry Ford. After presenting this fictitious threat, Coughlin’s paper called for action, in the form of a “crusade against the anti-Christian forces of the red revolution” — a call that was answered, in New York and Boston, by a new organization, the Christian Front. Its members were among the most enthusiastic participants in a 1939 pro-Hitler rally that packed Madison Square Garden, where the leader of the German-American Bund spoke in front of an enormous portrait of George Washington flanked by swastikas.

The Bund took a mortal hit that same year — its leader was caught embezzling — but the Christian Front soldiered on. In 1940, a New York chapter was raided by the F.B.I. for plotting to overthrow the government. The organization survived, and throughout World War II carried out what the New York Yiddish paper The Day called “small pogroms” in Boston and New York that left Jews in “mortal fear” of “almost daily” beatings. Victims who complained to authorities, according to news reports, were “insulted and beaten again.” Young Irish-Catholic men inspired by the Christian Front desecrated nearly every synagogue in Washington Heights. The New York Catholic hierarchy, the mayor of Boston and the governor of Massachusetts largely looked the other way.