In the Oval Office one day last week, President Trump renewed his no-holds-barred attack on the nation’s oldest university. “They’re totally antisemitic at Harvard,” he declared. Just 10 hours later, he posted an image of himself striding down a street with the caption, “He’s on a mission from God and nothing can stop what is coming.” Shown in the shadows, watching with approval, was a cartoon figure commonly seen as an antisemitic symbol. The appearance of the figure, the alt-right mascot Pepe the Frog, was the latest example of Mr. Trump’s extensive history of amplifying white supremacist figures and symbols,...