WASHINGTON, United States (AFP) — 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 Act (VRA).

The decision leaves the core of the VRA intact but narrows how it can be applied, and will be seen by civil rights activists as a blow to a landmark law that has already been weakened by previous rulings over the past decade.

Democracy Docket, a voting rights media platform, said the ruling could help secure 27 extra Republican seats, cementing the party’s control in the US House of Representatives for at least a generation.

“Without racial protections, maps could be redrawn with almost no limits,” the outlet reported.

It was not immediately clear how much the ruling will affect November’s elections, with primaries already underway and legal challenges likely to delay any changes.

Republicans are nonetheless expected to push for new maps in states where time and the law permit.

Writing for the majority, Justice Samuel Alito said that compliance with the VRA “could not justify” the use of race in the case, arguing that Section 2 of the Act does not require states to draw districts primarily on racial lines.

“That map is an unconstitutional gerrymander, and its use would violate the plaintiffs’ constitutional rights,” he said, referring to the group of non-black voters who brought the case.

The ruling marks a significant shift in how courts interpret the balance between preventing racial discrimination and adhering to constitutional guarantees of equal protection, raising the bar for when race can be considered in redistricting.

Both the majority ruling and the dissent were read from the bench, a rare move in the high court.

Justice Elena Kagan warned that the decision would have sweeping consequences, saying it risked letting states “systematically dilute minority citizens’ voting power” without legal redress.

The case centred on an electoral map redrawn after the 2020 census to add a second district in which black voters formed a majority, following court challenges that the previous map diluted minority voting strength.

That revised map was then challenged by a group of non-black voters who argued it relied too heavily on race. The Supreme Court ultimately sided with those challengers.

The implications could extend far beyond a single state. Legal analysts say the ruling may make it harder to create or maintain majority-minority districts — a key tool used for decades to ensure representation for black and other minority voters.

Such districts have often favoured Democratic candidates, meaning the decision could give Republicans an advantage in closely contested House races in November.

“After today, those districts exist only on sufferance, and probably not for long,” Kagan said.

Chuck Schumer, the top Democrat in the US Senate, called the decision a “devastating blow” to the VRA.

“Today, the Supreme Court turned its back on one of the most sacred promises in American democracy — the promise that every voice counts,” Schumer said in a statement.

The case comes amid an intensifying national battle over redistricting, with both Republican- and Democratic-led states seeking to redraw boundaries in ways that could shift the balance of power in Congress.

Section 2 of the Voting Rights Act — the provision at the centre of the case — was designed to prevent voting practices that dilute minority influence, even without explicit evidence of discrimination.

But the court’s conservative majority has increasingly signalled discomfort with race-conscious remedies, suggesting they must have limits under what conservative Justice Clarence Thomas, who is black, described as a “colour-blind” constitution.