Monday’s decision continued a trend at the Supreme Court, where justices have found that racial considerations improperly tainted redistricting by GOP-led legislatures in Virginia, Alabama and North Carolina. (Michael S. Williamson/The Washington Post)

The Supreme Court ruled Monday that North Carolina’s Republican-controlled legislature relied on racial gerrymandering when drawing the state’s congressional districts, a decision that could make it easier to challenge other state redistricting plans.

The decision continued a trend at the court, where justices have found that racial considerations improperly tainted redistricting decisions by GOP-led legislatures in Virginia, Alabama and North Carolina. Some cases involved congressional districts, others legislative districts.

The states contended that their efforts were partisan moves to protect their majorities, which the Supreme Court in the past has allowed, rather than attempts to diminish the impact of minority voters, which are forbidden.

But the justices declared that North Carolina had relied too heavily on race in its efforts to “reshuffle,” in the words of Justice Elena Kagan, voters in two congressional districts. They were unanimous in rejecting one of the districts and split 5 to 3 on the other.

(Daron Taylor/The Washington Post)

“This is a watershed moment in the fight to end racial gerrymandering,” said former attorney general Eric H. Holder Jr., who is part of a Democratic effort focused on redistricting. “North Carolina’s maps were among the worst racial gerrymanders in the nation. Today’s ruling sends a stark message to legislatures and governors around the country: Racial gerrymandering is illegal and will be struck down in a court of law.”

North Carolina leaders said the court had made the rules regarding redistricting even murkier. Lawmakers are required to consider race when drawing legislative lines so that minorities have a chance to elect candidates of their choice when the numbers are there. But the court has said racial considerations cannot predominate when drawing the districts.

“We have the utmost respect for the Supreme Court, but it is challenging for our lawmakers to draw congressional districts that the courts will accept when the courts regularly change the rules state legislatures must follow when drawing them,” said Amy Auth, a spokeswoman for North Carolina Senate Leader Phil Berger.

Kagan, writing for the court’s majority, criticized lawmakers for increasing the share of black voters in the two districts to more than 50 percent, when experience had shown that was not necessary in order for minority voters to elect representatives of their choice. The Voting Rights Act does not require that, she said, and the real goal might have been to make surrounding districts safer for white candidates.

Justices will not “approve a racial gerrymander whose necessity is supported by no evidence and whose raison d’être is a legal mistake,” Kagan wrote.

That is one of the most important parts of the decision, said Richard Pildes, an election law expert at New York University. The ruling “opened up a new avenue of constraint by holding that if white voters now vote for candidates black voters prefer at high enough rates, the intentional creation of majority-black districts is no longer required — and indeed, is unconstitutional,” Pildes wrote in an analysis of the decision.

Redistricting is part of a bare-knuckled fight in North Carolina, where Democrats and Republicans uneasily share political power.

Last week, the Supreme Court said it would not review a lower court’s ruling that a sweeping 2013 rewrite of the state’s voting laws was an unconstitutional attempt to diminish the power of African American voters. The state’s Republican legislative leadership had asked the Supreme Court to get involved, but the Democratic governor and attorney general said they did not want to defend the law.

In the case involving the congressional districts, a lower court had found them improperly drawn, and in 2016, elections were held in redrawn districts. Republicans maintained control of 10 of 13 districts.

The partisan battle over redistricting has raged in North Carolina for decades; Kagan noted that one of the districts in question was “making its fifth(!) appearance before this Court.”

The justices unanimously agreed that District 1, anchored in the northeastern part of the state and stretching into the black neighborhoods of Durham, was improperly drawn.

But the court split on District 12, which Kagan described as a “snakelike” district in the south-central section of the state that includes part of Charlotte.

In an unusual alliance, conservative Justice Clarence Thomas joined Kagan and the court’s other liberals — Ruth Bader Ginsburg, Stephen G. Breyer and Sonia Sotomayor — in saying the Voting Rights Act did not justify lawmakers’ actions.

Kagan noted that lawmakers redrew District 12 so that it had 35,000 more blacks of voting age and 50,000 fewer whites. She said the state’s contention that it was simply adding Democrats was contradicted by testimony that only 18 percent of the region’s white Democrats ended up in the district, while 65 percent of black Democrats did.

Chief Justice John G. Roberts Jr. and Justices Anthony M. Kennedy and Samuel A. Alito Jr. objected. Alito wrote that “District 12’s borders and racial composition are readily explained by political considerations and the effects of the legislature’s political strategy.”

Alito complained that in a previous decision, the court insisted that those challenging districts present alternative maps showing that the legislature could achieve its political goals without “the racial effects giving rise to the racial gerrymandering allegation.” The majority relieved the challengers of providing such a map in this case, he said.

“A precedent of this Court should not be treated like a disposable household item — say, a paper plate or napkin — to be used once and then tossed in the trash,” Alito wrote.

Kagan, in return, said Alito and the dissenters accepted the word of the North Carolina legislature rather than the three-judge panel that heard evidence in the case and found the congressional districts unacceptable.

The case is Cooper v. Harris. It was argued before Justice Neil M. Gorsuch joined the court, and he took no part in the decision.