Jennifer Burns, an assistant professor of history at Stanford, is the author of “Goddess of the Market: Ayn Rand and the American Right.”

EARLY in his Congressional career, Paul D. Ryan, the Wisconsin representative and presumptive Republican vice-presidential nominee, would give out copies of Ayn Rand’s book “Atlas Shrugged” as Christmas presents. He described the novelist of heroic capitalism as “the reason I got into public service.” But what would Rand think of Mr. Ryan?

While Rand, an atheist, did enjoy a good Christmas celebration for its cheerful commercialism, she would have scoffed at the idea of public service. And though Mr. Ryan’s advocacy of steep cuts in government spending would have pleased her, she would have vehemently opposed his social conservatism and hawkish foreign policy. She would have denounced Mr. Ryan as she denounced Ronald Reagan, for trying “to take us back to the Middle Ages, via the unconstitutional union of religion and politics.”

Mr. Ryan’s youthful, feverish embrace of Rand and his clumsy attempts to distance himself from her is more than the flip-flopping of an ambitious politician: it is a window into the ideological fissures at the heart of modern conservatism....