Everything seemed to be falling apart. After his election to the presidency in November 1932, Franklin D. Roosevelt received a talkative friend. If Roosevelt could rescue America from the Great Depression, the friend said, then he would be remembered as the greatest of presidents; if he failed, he would go down as the worst. There were alternatives to democratic capitalism afoot in the world: fascism in Germany and Italy, communism in Russia. Listening to his visitor, Roosevelt was matter-of-fact. “If I fail,” he said, “I shall be the last one.”

And so, to some extent and some degree, we’ve been here before: a sense of crisis, of crumbling order, of facing destructive forces that may prove beyond our control. We survived the 1930s not least because Roosevelt did not fail, and thus was of course not the last president. Among the weapons he deployed in his wars to save capitalism in the 1930s and democracy itself in the first years of the ’40s, one might seem trivial, but isn’t: the radio.

It can be difficult, in our own media-saturated age, to recall the revolutionary nature of radio and later of television. As Richard Hofstadter, the Columbia historian, wrote, the “growth of the mass media of communication and their use in politics have brought politics closer to the people than ever before and have made politics a form of entertainment in which the spectators feel themselves involved.” He added, “Mass communications have aroused the mass man.”

In the beginning of each great electronic media transformation — radio and television, and now social media — there have been anxieties that mass communication would enable demagogy and trivialize governance.

In the first two cases, chiefly because of Roosevelt and his educational use of the Fireside Chats, and John F. Kennedy and Ronald Reagan with their TV performances, that concern proved to be overblown. Today’s case is still an open question: Twitter could, in the right and calm hands, be a tool for presidential good. Of course, people worry that in the hands of President Trump, it is instead a tool for division and social erosion. ...