When Robert F. Kennedy Jr. announced his primary challenge to Joe Biden in April, the media shrugged him off as a meme candidate who would soon collide with political reality. Two months later, Kennedy is polling as high as 20% and his campaign is gaining momentum. And the media — unwilling to let Biden be primaried by this pedigreed populist — have decided it’s time to step in. And step in they have. Over the past weeks, major print and broadcast organizations have laid the foundations for a powerful narrative designed to hobble Kennedy’s campaign before a single vote is...