What’s in a name? Franklin Delano Roosevelt called himself a Christian, a Democrat, and a liberal. He did not call himself a democratic socialist, or any other kind of socialist. He was, in fact, no socialist at all. Nor was he a conservative or a reactionary, although many on the socialist and communist left charged that he was—including the Communist Party USA, which attacked his New Deal for a time (until Moscow’s political line changed) as American “masked fascization.”

The only Americans who considered Franklin Roosevelt a socialist were right-wing Republicans. “The New Deal is now undisguised state socialism,” Senator Simeon D. Fess of Ohio declared in 1934. “Roosevelt is a socialist, not a Democrat,” Congressman Robert Rich of Pennsylvania announced on the House floor a year later. Roosevelt scoffed at such talk, but in 1939 he paused to present a very concise political dictionary of his own. “A radical,” he told the New York Herald Tribune, “is a man with both feet firmly planted—in the air.” A conservative, he continued, “never learned to walk forward”; a reactionary walked backward in his sleep. A liberal, though, used legs and hands “at the behest—at the command—of his head.” The metaphor was poignant coming from him, but it also emphasized his point: In the face of all adversity, he was every inch a liberal.

In the 1936 election, FDR masterfully ran as an unabashed liberal and at the same time completely outmaneuvered the left and would-be populists like Louisiana Governor Huey Long, who, before his assassination, planned to challenge Roosevelt in the campaign on a “Share Our Wealth” platform. As Seymour Martin Lipset and Gary Marks related in It Didn’t Happen Here: Why Socialism Failed in the United States, the Great Depression “presented American radicals with their greatest opportunity to build a third party since World War I.” But Roosevelt’s New Deal, in its improvisational way, offered a triumphant liberal alternative.

The election of 2016 showed how confused these old labels and distinctions have become. The socialist senator Bernie Sanders, for example, rallying his supporters with a speech at Georgetown University in November 2015, offered a surprising definition of socialism, which consisted of a paean to FDR and the social protections ushered in by the New Deal. “Almost everything he proposed, almost every program, every idea, was called socialist,” Sanders said—as if the right-wing name-calling was the rightful definition.

Somewhere the ghost of FDR burst out laughing, while the ghost of one of Sanders’s other heroes, Eugene V. Debs, scratched his head. ...