Robert F. Kennedy Jr. is not an intimidating political opponent. Or at least not on the surface. Yes, he is a scion of Camelot named for his father, a patron saint of American liberalism. But beyond the Kennedy factor, everything about him screams amusing sideshow rather than serious contender.

His main contribution to public life over the last few decades has been as the country’s most prominent antivaxxer — a fringe role almost by definition.