A federal court in Cincinnati struck down Ohio’s congressional map Friday, finding the state legislature’s Republican majority drew an “unconstitutional partisan gerrymander” in 2012 that amplified GOP power far beyond the party’s actual popularity.

The special three-judge court unanimously ordered Ohio to adopt a new map by June 14, well ahead of the 2020 congressional election. But the 301-page opinion might best be seen as an unofficial brief to the U.S. Supreme Court, which is expected to issue a decision before July that...