The Supreme Court on Monday struck down two North Carolina congressional districts, ruling that lawmakers had violated the Constitution by relying too heavily on race in drawing them.
The court rejected arguments from state lawmakers that their purpose in drawing the maps was not racial discrimination but partisan advantage.
The decision was the court’s latest attempt to solve a constitutional puzzle: how to disentangle the roles of race and partisanship when black voters overwhelmingly favor Democrats. The difference matters because the Supreme Court has said that only racial gerrymandering is constitutionally suspect.
Election law experts said the ruling would make it easier to challenge voting districts based partly on partisan affiliations and partly on race.
“This will lead to many more successful racial gerrymandering cases in the American South and elsewhere,” said Richard L. Hasen, a law professor at the University of California, Irvine.
Democrats welcomed the ruling.
“This is a watershed moment in the fight to end racial gerrymandering,” Eric H. Holder Jr., the former attorney general and the chairman of the National Democratic Redistricting Committee, said in a statement. “North Carolina’s maps were among the worst racial gerrymanders in the nation.”
Conservatives complained that the Supreme Court had succeeded only in making the law murkier.
“The Supreme Court says race can be a factor in redistricting, but not the predominant factor, a rule that is so vague, so broad and so lacking in a definable legal standard that it is not really a rule at all,” Hans von Spakovsky, a lawyer with the Heritage Foundation, said in a statement.
The ruling on Monday was the second Supreme Court victory for North Carolina Democrats this month. Last Monday, the justices declined to hear an appeal of a decision that had struck down parts of a restrictive North Carolina voting law that, among other things, tightened voter identification requirements and cut back on early voting.
A federal appeals court had ruled that the restrictions were an unconstitutional effort to “target African-Americans with almost surgical precision.”
In their decision this week, the justices were unanimous in rejecting District 1, in the northeastern part of the state. After the 2010 census, lawmakers increased the district’s black voting-age population to 52.7 percent from 48.6 percent.
Justice Elena Kagan, writing for the court, said black voters, in coalitions with others, had been able to elect their preferred candidates even before the redistricting. Adding additional black voters to the district, she wrote, amounted to an unconstitutional racial gerrymander.
The court divided, 5 to 3, in rejecting District 12, in the south-central part of the state. Lawmakers increased the number of black voters to 50.7 percent from 43.8 percent. “To be specific, the new District 12 had 35,000 more African-Americans of voting age and 50,000 fewer whites of that age,” Justice Kagan wrote.
The five-justice majority for the part of the decision concerning District 12 was an unusual coalition. Justices Clarence Thomas, Ruth Bader Ginsburg, Stephen G. Breyer and Sonia Sotomayor joined Justice Kagan’s majority opinion.
In defending the new district lines for District 12, state lawmakers said they had meant to secure a partisan advantage for Republicans, a lawful goal. But Justice Kagan said race could predominate even if legislators had mixed motives.
“The sorting of voters on the grounds of their race remains suspect even if race is meant to function as a proxy for other (including political) characteristics,” she wrote, in a passage that some election law experts said would make it easier to challenge asserted racial gerrymanders.
In a concurrence, Justice Thomas said he agreed that the trial court had not committed a clear error in finding that “race was North Carolina’s predominant motive in drawing the district.”
In dissent, Justice Samuel A. Alito Jr., joined by Chief Justice John G. Roberts Jr. and Justice Anthony M. Kennedy, wrote that the challengers had not shown that race rather than partisanship motivated the drawing of the district.
The two factors are highly correlated, Justice Alito wrote.
“This phenomenon makes it difficult to distinguish between political and race-based decision making,” he wrote. “If around 90 percent of African-American voters cast their ballots for the Democratic candidate, as they have in recent elections, a plan that packs Democratic voters will look very much like a plan that packs African-American voters.”
Allowing race-based challenges, Justice Alito wrote, creates the danger that the federal courts will be transformed into “weapons of political warfare.”
“If the majority party draws districts to favor itself, the minority party can deny the majority its political victory by prevailing on a racial gerrymandering claim,” he wrote. “Even if the minority party loses in court, it can exact a heavy price by using the judicial process to engage in political trench warfare for years on end.”
Justice Alito also criticized the challengers for not proposing an alternative map, which he said was required by an earlier decision. “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,” he wrote.
Justice Kagan responded that while proposed alternative maps might be helpful to courts evaluating racial gerrymandering claims, they were not required. All a plaintiff challenging a voting district needs to prove, she wrote, quoting an earlier decision, is that “race (not politics) was the ‘predominant consideration in deciding to place a significant number of voters within or without a particular district.’”
Justice Neil M. Gorsuch did not participate in the case, which was argued before he joined the court.
The decision, Cooper v. Harris, No. 15-1262, affirmed a decision from the Federal District Court in Greensboro, N.C.