James Comey’s new memoir is already a pre-publication media hit, but the former FBI director’s tell-all assault on President Donald Trump also poses major risks—not only for Comey’s personal reputation but for the wider Russia investigation he helped launch. Trump and his allies bombarded Comey with insults Friday, targeting his credibility and seeking to undermine his image as a nonpartisan law enforcement professional. On Twitter Friday, Trump called Comey an “untruthful slime ball,” while White House press secretary Sarah Sanders branded him a “discredited partisan hack.” Other critics accused Comey of cashing in on his celebrity to an unseemly degree....