Washington DC - The Supreme Court on Thursday rejected a challenge to a congressional map in South Carolina that civil rights groups said was improperly drawn along racial lines.

The case touching on the thorny issues of race and politics could help determine whether Democrats or Republicans control the House of Representatives next year.

In a 6-3 vote, the conservative-dominated top court ruled that a congressional district map drawn by the Republican-majority legislature in South Carolina was not an unconstitutional racial gerrymander.

The case was one of several legal battles involving alleged racial gerrymandering – the manipulation of electoral maps that dilutes the voting power of minorities – winding their way through US courts.

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In the South Carolina case, a three-judge federal panel ruled unanimously in January 2023 that a congressional district redrawn after the 2020 census was an illegal racial gerrymander and ordered it to be reconfigured before the November 2024 election.

The redrawn congressional map moved 60% of the Black residents of the coastal city of Charleston – nearly 30,000 people – from one district into another, which already had a Black majority.

African Americans tend to vote overwhelmingly Democratic, and six of the current House members from South Carolina are white, while one is Black.

The South Carolina legislature challenged the district court's ruling and the case ended up before the Supreme Court in October.

The nation's highest court, in an opinion written by archconservative Justice Samuel Alito, said the lower court's finding that race was the predominating factor in redrawing the district map was "clearly erroneous."

"Redistricting is an inescapably political enterprise," Alito said, and "a legislature may pursue partisan ends when it engages in redistricting."

"Where race and politics are highly correlated, a map that has been gerrymandered to achieve a partisan end can look very similar to a racially gerrymandered map," he said.

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Alito was joined by the court's other five conservatives, while the three liberal justices dissented.

Justice Elena Kagan called the ruling "seriously wrong" and said it would pave the way for more lawmakers to use race to achieve partisan goals.

"And so this 'odious' practice of sorting citizens, built on racial generalizations and exploiting racial divisions, will continue," Kagan said.

Republicans currently hold a slim two-seat majority in the House, and an increase in the number of Black-majority districts could tip the balance in November's congressional elections when all 435 House seats will be up for grabs.

Adriel Cepeda Derieux, deputy director of the voting rights project of the American Civil Liberties Union, a party to the case, criticized the ruling as an "affront to Black voters, democracy, and precedent."

"South Carolina's Legislature carved Black voters out of Congressional District 1 for the sake of partisan advantage and weakening their voting power," Derieux said.

South Carolina's is one of a number of congressional redistricting cases being fought in the courts following the 2020 census.

A redrawn congressional map in Louisiana also faced a legal challenge on the grounds it resulted in just one Black-majority district, although African Americans make up 30% of the southern state's population.

Two conservative Supreme Court justices sided with the liberals in that case and restored a second Black-majority district.