What an enormous abdication. The Supreme Court ducked its first Second Amendment case in a decade on Monday, and the only plausible explanation is that Chief Justice John Roberts wanted to avoid becoming a target of vengeful Senate Democrats.

In an unsigned per curiam opinion, the Chief joined the four liberals and a (conflicted) Justice Brett Kavanaugh in declaring moot a challenge to New York City’s onerous gun regulation (New York State Rifle & Pistol Association v. City of New York). At issue is a New York City rule that...