Academic historians have a testy relationship with popular history writers. Many a history professor has produced a weighty, path-breaking tome, only to find that when some nonfiction writer produces a beach read on the same topic, it outsells the academic title by thousands of copies. But some of the most popular history authors do thoroughly consult academic histories, integrating them into gripping narratives preferred by the reading public.

Consider J.D. Dickey’s “American Demagogue: The Great Awakening and the Rise and Fall of Populism.” The title made me wince. I assumed Mr. Dickey, author of a previous book on Sherman’s March to the Sea, would somehow juxtapose the famous evangelist George Whitefield with Donald Trump. Perhaps he would use them to show how religion and politics give cover to charlatans who infest both fields.

But Mr. Dickey does much more than this. Yes, he advances the obligatory comparison of Whitefield and President Trump at the outset. He largely drops the theme of demagoguery for the rest of the book, however, which becomes a study of a more profitable theme: populism. Indeed, Mr. Dickey can’t quite decide if Whitefield was a demagogue, since the term connotes insincerity. Some scholars have suggested that the great evangelist was a shallow manipulator, to be sure. But Mr. Dickey is too impressed by the transformative power of Whitefield’s preaching, and by his enduring friendship with Benjamin Franklin, to class Whitefield among great American demagogues, from Huey Long to Donald Trump.

What Mr. Dickey’s book contemplates is the curious tendency of populism to veer off in radical directions that its originators (including Whitefield) do not intend. Mr. Dickey’s Great Awakening produced many unintended consequences. Among them: the American Revolution.