Donald Trump and Bernie Sanders share an upbringing in New York’s outer boroughs and a repugnance for trade deals, but the similarities pretty much end there. Sanders routinely inveighs against “corporate America”; Trump is an executive of almost 500 business entities, more than 200 of which are named after himself. Trump lends his name to a line of power ties and cuff links; the adjective most often applied to Sanders’s wardrobe is “rumpled.” Yet journalists routinely refer to both men as “populists.” How can a word that purports to describe both a proud socialist and an arrogant billionaire have any meaning?

For half a century, most presidential campaigns have featured one or more “populists” from the right, the left or somewhere in between. In 1968, reporters and academics pasted the label on George Wallace, whose campaign literature asked, “Can a former truck driver married to a dime-store clerk and son of a dirt farmer be elected president?” In 1972, Time dubbed George McGovern a “prairie populist” because he had a modest plan to redistribute wealth and hailed from the rural heartland. In 1996, The Atlantic observed that Pat Buchanan’s “hard-right-wing populism ... may be the shape of politics to come.” In 2012, The Hill announced, “Obama cranks up populist pitch” after the president, who previously shied away from us-versus-them talk, called for higher taxes on the rich.

There was a time when “populist” meant something more specific. The word originated with the decidedly left-wing People’s Party that emerged in the Midwest and the South amid the economic turmoil and rampant inequality of the 1890s. Journalists who knew some Latin started calling them “Populists” as a shorthand, and the name stuck. Those uppercase Populists championed small farmers and wage-earners who thought “the money power” — banks and industrial corporations — had seized control of both America’s economy and its government. The party called for nationalizing the railroads, breaking up the trusts and strengthening labor unions. At times, their leftism toppled over into paranoia; to explain society’s ills, they invoked “a vast conspiracy against mankind,” engineered by a plutocratic cabal.

The Populists joined forces with the Democrats for the 1896 election and collapsed soon afterward. The word “populist” mostly disappeared into academic studies until the 1950s, when Joseph McCarthy, a previously obscure Republican senator from Wisconsin, rose to promi­nence with his claims that Communists had infiltrated positions of power in the American government and military. Many targets of his rants were members of the East Coast liberal intelligentsia.

Millions of Americans cheered on McCarthy’s crusade — horrifying liberal intellectuals like the historian Richard Hofstadter and the sociologist Daniel Bell, who reached back to the precedent of the Populists to understand the roots of the new anti-elitist fervor. In McCarthy, they heard echoes of the original Populists’ conspiracy theorizing — only this time, the perpetrators were well-born “pinkos” or “reds.”

Other scholars dismissed the comparison as deeply unfair, but Hofstadter’s effort to link the Populists to what he called “the paranoid style” of the new right resonated. And so “populism” began to morph into a handy tool of journalistic discourse. ...