WASHINGTON - The Supreme Court ruled Monday that electoral districts must have a majority of African-Americans or other minorities to be protected by a provision of the Voting Rights Act.

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."

Chief Justice John Roberts and Justice Samuel Alito signed onto Kennedy's opinion. Justices Antonin Scalia and Clarence Thomas agreed with the outcome of the case.

The four liberal justices dissented. A district like the one in North Carolina should be protected by federal law "so long as a cohesive minority population is large enough to elect its chosen candidate when combined with a reliable number of crossover voters from an otherwise polarized majority," Justice David Souter wrote for himself and Justices Stephen Breyer, Ruth Bader Ginsburg and John Paul Stevens.

The decision complicates matters for southern Democrats who will redo political boundaries after the next census.

Democrats have sought to create districts in which African-Americans, though not a majority, still were numerous enough to determine the outcome of elections with the help of small numbers of like-minded white voters. Those districts could be challenged under Monday's decision.

In another election-related case, the court let stand an appeals court decision that invalidated state laws regulating the ways independent presidential candidates can get on state ballots.

Arizona, joined by 13 other states, asked the court to hear its challenge to a ruling throwing out its residency requirement for petition circulators and a June deadline for submitting signatures for independent candidates in the November presidential elections.

Independent presidential candidate Ralph Nader sued and won a favorable ruling from the 9th U.S. Circuit Court of Appeals in San Francisco.