u.s._supreme_court.jpgWASHINGTON (AP) _ The Supreme Court ruled Monday that a part of the Voting Rights Act aimed at helping minorities elect their preferred candidates only applies in electoral districts where minorities make up more than half the population.

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.

Although 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.

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 that 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.

Kennedy said that, absent prohibitions like North Carolina's rule against crossing county lines, “states that wish to draw crossover districts are free to do so.'' But they are not required, he said.

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.

Civil rights groups that urged the court to uphold the North Carolina plan said such districts help to diminish racially polarized voting over time because the candidate who is the choice of black or Hispanic voters must draw some white support to win election.

In April, the court will hear a more significant challenge to another provision of the Voting Rights Act, requiring all or parts of 16 states with a history of racial discrimination to get approval before implementing any changes in the way elections are held.

The court's familiar ideological split in this case strongly suggests that Kennedy could hold the key to the outcome in the April case as well, said Nathaniel Persily, an election law expert at Columbia University.

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.

The cases are Bartlett v. Strickland, 07-689, and Brewer v. Nader, 08-648.