The anxiety began well before the Cleveland convention, where the candidate of the “Forgotten Men,” the one who declared Americans “the greatest Race on the face of this old Earth,” seemed likely to clinch his party’s presidential nomination. Doremus Jessup, the protagonist of Sinclair Lewis’s 1935 novel “It Can’t Happen Here,” sees something dark and terrible brewing in American politics — the potential for “a real fascist dictatorship” led by the up-and-coming populist candidate Berzelius Windrip. Friends scoff at this extravagant concern. “That couldn’t happen here in America, not possibly!” they assure him. But Jessup, a small-town Vermont newspaper editor