The decision gives lawmakers until Aug. 1 to approve new district maps, which would need to be signed by Democratic Governor Gretchen Whitmer.
If either legislators fail to do so, or if the court finds the new district lines are similarly unconstitutional, the judges said they would draw the maps themselves, likely with the help of a third-party expert.
“Today, this court joins the growing chorus of federal courts that have, in recent years, held that partisan gerrymandering is unconstitutional,” U.S. District Judge Eric Clay wrote for a unanimous three-judge panel.
In late March, the U.S. Supreme Court heard arguments on whether such gerrymandering violates the U.S. Constitution in a case stemming from the electoral maps in Maryland and North Carolina.
Partisan gerrymandering is the process by which one party draws legislative districts to weaken the other party’s voters. The lines are typically redrawn once a decade after the U.S. census, and in many states the party in power controls the process.
The court also ordered Michigan to hold special state Senate elections next year, rather than in 2022 as scheduled, in any gerrymandered districts.
The decision is a boon for Democrats, who failed last year to win a majority of the seats in the state House of Representatives, state Senate or the state’s U.S. congressional delegation despite winning the overall popular vote statewide in all three cases.
Republican lawmakers, who intervened in the case, had unsuccessfully argued that the Michigan case should have been put on hold until the Supreme Court rules. They are likely to appeal Thursday’s decision.
Reporting by Joseph Ax; Editing by Alistair Bell