President Joe Biden is all but assured to win the Democratic presidential nomination in 2024 as the incumbent. But one of his Democratic challengers, anti-vaxxer Robert F. Kennedy Jr., is gaining a surprising amount of traction for someone who the national party views as a fringe candidate. Kennedy, who announced his candidacy in April, is the nephew of former President John F. Kennedy and son of former Attorney General Robert F. Kennedy. He backed Hillary Clinton for president in 2016. But that’s where his relationship with the Democratic political establishment ends. Disavowed by members of his own family, the previously...