WASHINGTON—Maryland’s Democratic attorney general said Thursday he would appeal a court order to redraw a congressional district found to disadvantage Republicans, increasing the odds of a new Supreme Court battle over legislative maps that could reshape political lines before the 2020 elections.

Already pending before the justices is a ruling from a special federal court in North Carolina, which found that state’s congressional map an unconstitutional gerrymander drawn by majority Republicans to disenfranchise Democrats.

In Thursday’s court filing, Maryland Attorney General Brian Frosh said the parties would file their legal briefs by Dec. 18, allowing the justices to schedule the case for their April sitting and issue a ruling by June.

The Supreme Court has twice considered the Maryland case before, in both instances returning it to a lower court rather than confront the core issue: Does the Constitution permit legislators to draw maps that amplify the majority’s political power far beyond its share of the vote?

The justices most recently dodged that question in June, declining in the Maryland case—and a separate challenge to Wisconsin’s state Assembly map—to set forth constitutional standards. It also told a North Carolina federal court to reconsider its finding that the state’s congressional map, drawn by majority Republicans in the state legislature, disenfranchised Democrats. That ruling has now been reaffirmed and sent back to the Supreme Court.