Washington in 2016 saw one of most audacious dirty tricks in political history: the Donald Trump-Russia collusion claim. Now it’s happening again—same partisans, same media; new conspiracy, new victims, including the National Rifle Association.

Remember how the Trump-Russia trick played? Hillary Clinton’s campaign hired opposition-research firm Fusion GPS to compile a dossier of salacious Trump allegations. The Fusion team delivered it to the Federal Bureau of Investigation, then, when briefing reporters, cited the bureau’s...