The case will be argued in the fall and will be decided well in advance of the redrawing of political districts that will follow the 2010 Census.
"The answer to this question will affect the voting rights of minorities throughout the country," North Carolina Attorney General Roy Cooper said in asking the court to take the case.
The dispute involves North Carolina legislative districts in which black voters make up less than 50 percent of the population, but still are numerous enough to elect a black candidate who has some support from white voters.
Blacks make up 39 percent of the voting-age population in the district at the heart of the lawsuit filed by county commissioners. The district has elected a black state lawmaker since 1992.
The issue is whether such districts are protected by a provision of the federal Voting Rights Act. The North Carolina Supreme Court ruled that the landmark civil rights law does not apply to districts where a minority accounts for less than half the population.
Civil rights groups said the ruling would allow lawmakers to pack black voters into fewer districts, thus reducing their voting strength.
In urging the justices to intervene, the NAACP and other groups said that racially polarized voting has declined in recent decades, allowing black voters to be heard even when they lack a voting majority in some areas.
The Supreme Court has declined several times over the past 20 years to settle the matter. Most recently, the court sidestepped the issue in its decision in 2006 on Texas' Republican-inspired mid-decade redistricting.
Lawyers for the county commissioners said the 50-percent rule is a "bright line" that lawmakers can easily apply.