The US Supreme Court on Wednesday sharply limited the use of race in drawing electoral districts, in a decision that could reshape congressional maps nationwide and boost Republican prospects ahead of midterm elections.

In a 6-3 ruling split along ideological lines, the conservative-dominated court struck down a map that creates a second majority-black district in Louisiana, finding it amounted to an unconstitutional racial gerrymander despite being drawn to comply with the 1965 Voting Rights...