WASHINGTON (Reuters) - The U.S. Supreme Court on Wednesday told a lower court in a voting rights case to reassess whether Virginia’s Republican-led legislature unlawfully tried to dilute the clout of black voters when it drew a series of state legislative districts.

A general view of the U.S. Supreme Court building in Washington, U.S., November 15, 2016. REUTERS/Carlos Barria

A group of voters who filed the legal challenge said the lawmakers improperly considered race as a factor when mapping boundaries of state House of Delegates voting districts. A federal district court in 2015 upheld 12 districts, but the justices directed a three-judge panel of the lower court to take a fresh look at the propriety of 11 of those.

The case is one of a number of lawsuits accusing Republicans of taking steps at the state level to disenfranchise black and other minority voters who tend to back Democratic candidates.

The voters who brought the lawsuit accused Republicans of packing black voters into certain districts to diminish their voting power and make surrounding districts more white and more likely to support Republicans.

The eight-member court was unanimous in ordering the further review of the 11 districts in the opinion written by conservative Justice Anthony Kennedy, and voted 7-1 to uphold the other district, with conservative Justice Clarence Thomas asserting that the 12th district should not have been upheld.

Kennedy wrote that the lower court did not sufficiently analyze the consideration of race during the redistricting process.

The lower court had said it needed to look at race only if the district in question was not drawn based on “traditional redistricting principles.” The high court faulted that approach, saying that a regularly drawn map does not tell the full story.

“But if race for its own sake is the overriding reason for choosing one map over others, race may still predominate,” Kennedy wrote.

“For these reasons, a conflict or inconsistency between the enacted plan and traditional redistricting criteria is not a threshold requirement or a mandatory precondition in order for a challenger to establish a claim of racial gerrymandering,” he added.

Gerrymandering refers to manipulating the boundaries of a voting district in order to favor a particular party.

The court sidestepped the issue of whether the 11 districts had been drawn by Republicans in a way that violated the voters’ rights under the 14th Amendment of the U.S. Constitution, which requires that state legislature districts be drawn based on population.

At issue was the state legislative map drawn by Republicans after the 2010 census.

Democrats have accused Republicans in Virginia and other states of crafting such legislative maps in a way that crams black and other minority voters into certain districts in order to reduce their overall sway in the state.

Race can be considered in redrawing boundaries of voting districts only in certain instances, such as when states are seeking to comply with the federal Voting Rights Act. That law protects minority voters and was enacted to address a history of racial discrimination in voting, especially in southern states.

In 2015, the Supreme Court ruled 5-4 to throw out a lower court’s decision upholding a Republican-backed state legislature redistricting plan in Alabama that crammed black voters into certain districts in a way critics claimed lessened their influence at the polls.

The Supreme Court has never said redistricting cannot be based on nakedly partisan aims like maximizing one party’s election chances.