The decision could make it harder for some minority candidates to win election and for southern Democrats, in particular, to draw friendly electoral boundaries after the 2010 Census.

The 5-4 decision, with the court's conservatives in the majority, came in the case of a North Carolina plan that sought to preserve the influence of African-American voters even though they made up just 39 percent of the population in a state legislative district.

While not a majority, the black voters were numerous enough to effectively determine the outcome of elections, the state argued in urging the court to extend the civil rights law's provision to the district. The case dealt with the section of the law that bars states from reducing the chance for minorities to "elect representatives of their choice."

But Justice Anthony Kennedy, announcing the court's judgment, said the court would not extend the law to those so-called crossover districts. The 50 percent "rule draws clear lines for courts and legislatures alike," Kennedy said in striking down a North Carolina legislative district.

In 2007, the North Carolina Supreme Court struck down the district, saying the Voting Rights Act applies only to districts with a numerical majority of minority voters. The district also violated a provision of the state constitution keeping district boundaries from crossing county lines, the court said.