The Democratic debate Tuesday revealed the best thing that Hillary Clinton has going for her: the weakness of her challengers.

Vermont Sen. Bernie Sanders came off as an elderly, dyspeptic Bilbo Baggins attending a British Labour Party meeting. Former Maryland Gov. Martin O’Malley sounded like an overly earnest, slightly too intense 1950s ad man making a bad pitch. Former Rhode Island Gov. Lincoln Chafee was as hapless as a puppy stranded in a hurricane. And former Virginia Sen. Jim Webb, probably sensing how inconsequential...