Elections have a way of blowing up partisan conceits—see what happened to the Democratic Party’s Electoral College “lock” in 2016. This year Democrats busted one of their own cherished myths by proving that Republican gerrymanders weren’t preventing them from retaking the House of Representatives. There’s a lesson here for voters and judges.

State legislatures have been drawing congressional boundaries to favor one party or another since America’s founding. During the 40 years of sustained Democratic control of the House in...