The Steele dossier has already become a thing of John le Carré-like intrigue—British spies, Kremlin agents, legal cutouts, hidden bank accounts. What all this obscures is the more immediate point: The dossier amounts to one of the dirtiest tricks in U.S. political history. It was perpetrated by Team Clinton and yielded a vast payoff for Hillary’s campaign.

The Democratic National Committee and the Clinton campaign hired the opposition-research firm Fusion GPS in April 2016 to dig up dirt on Donald Trump. Fusion in turn hired...