Bernie Sanders’s victory in New Hampshire on top of his tie in Iowa makes him a favorite for the Democratic presidential nomination. Hard to believe, but in a winnable race against President Trump the opposition may nominate a socialist who wants the government to control energy production and health care, who wants nationwide rent control, and who calls America a “racist society from top to bottom.” No wonder Democrats like James Carville are in agony.

The Vermont revolutionary’s victory portends a long primary battle, unless...