The 5-4 decision, with the court's conservatives in the majority, could make it harder for southern Democrats to draw friendly boundaries after the 2010 Census.

The court declined to expand protections of the landmark civil rights law to take in electoral districts where the minority population is less than 50 percent of the total, but strong enough to effectively determine the outcome of elections.

In 2007, the North Carolina Supreme Court struck down a state legislative district in which blacks made up only about 39 percent of the voting age population. The court said the Voting Rights Act applies only to districts with a numerical majority of minority voters.

Justice Anthony Kennedy, announcing the court's judgment, said that requiring minorities to represent more than half the population "draws clear lines for courts and legislatures alike. The same cannot be said of a less exacting standard."