The Supreme Court limited the reach of the Voting Rights Act on Monday, a decision that could make it harder for some minority candidates to win election when voting districts are redrawn.

In a 5-4 decision, the justices ruled that a portion of the law aimed at helping minorities elect their preferred candidates only applies in districts where minorities make up more than half the population.

The decision could make it more difficult for Democrats, particularly in the South and Southwest, to draw electoral boundaries friendly to black or Hispanic candidates following the 2010 Census.

The Mexican American Legal Defense and Educational Fund said the decision weakens minorities’ ability to use the Voting Rights Act to challenge alleged discrimination in electoral districts.

“The Supreme Court decision, if left unchecked, will make redistricting in 2011 and the cause of making districts reflect emerging Latino electoral strength much harder,” said Nancy Ramirez, MALDEF’s western regional counsel.

With the court’s conservatives in the majority, the court ruled that North Carolina erred when trying 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 state said the district should be protected by the section of the law that bars states from reducing the chance for minorities to “elect representatives of their choice.”

Justice Anthony Kennedy, announcing the court’s judgment, said the court had never extended the law to those so-called crossover districts and would not do so now.