WASHINGTON (AP) — Democrats and Republicans are poised for a Supreme Court fight about political line-drawing with the potential to alter the balance of power across a country starkly divided between the two parties.

The big question at the heart of next week's high court clash is whether there can be too much politics in the inherently political task of drawing electoral districts. The Supreme Court has never struck down a districting plan because it was too political.

The test case comes from Wisconsin, where Democratic voters sued after Republicans drew political maps in 2011 that entrenche...