WASHINGTON (CNN) - Jumping into the political thicket of voting district boundaries, the Supreme Court on Monday ruled such districts must have a majority of African-American or other minority voters to qualify for federal electoral protection.
The 5-4 decision gives states greater discretion when drawing up so called "crossover" boundaries, and could affect minorities' ability to elect their candidates of choice.
At issue was whether a minority group must constitute 50 percent or more of voters in an election district in order to receive protection from having their voting power reduced when boundaries are redrawn.
The case involves a North Carolina state district where African-American voters were 39 percent of the population. That was less than half, but still numerous enough to potentially elect an African-American candidate with some white support.
The justices rejected calls that states be compelled to consider those scenarios when creating such boundaries, under conflicted interpretations of the landmark 1965 Voting Rights Act and a provision known as Section 2.
"We decline now to expand the reaches of Section 2 to require, by force of law, the voluntary cooperation our society has achieved," wrote Justice Anthony Kennedy. "Only when a geographically compact group of minority voters could form a majority in a single-member district," do Constitutional protections apply, he wrote.
He was supported by Chief Justice John Roberts and justices Samuel Alito, Clarence Thomas and Antonin Scalia.
Kennedy noted the need for "workable standards" necessary for such a politically-charged process to function. The 50-percent threshold "draws clear lines for courts and legislatures alike," he said. "The same cannot be said of a less exacting standard that would mandate crossover districts."
Such districts are created by state legislatures, and involve political maneuvering and negotiation to a large extent. The political party in power generally has the upper hand shaping state and congressional boundaries favorable to their side.
Civil rights groups had feared a Supreme Court decision against them would weaken minority voters' clout, and mean states would be free to pack African-Americans into fewer voting districts.
Justice David Souter echoed their concerns, saying the court majority's interpretation of Section 2 was "disturbing by any measure and flatly at odds with the obvious purpose of the [Voting Rights] Act.
"A district may be a minority-opportunity district so long as a cohesive minority population is large enough to elect a chosen candidate when combined with a reliable number of crossover voters from an otherwise polarized majority," wrote Souter, who was backed by Justice John Paul Stevens, Ruth Bader Ginsburg and Stephen Breyer.
The more liberal members of the bench did not offer a specific number below the 50-percent mandate that they believe should automatically ensure federal protection for minority voters. Such discretion should be left to the political process to ensure all citizens could participate fairly in the democratic process, they said.
The immediate effect of Monday's ruling is that such "crossover" districts can now be challenged in courts. That could make it harder for some boundaries to survive legal scrutiny. The area in question was District 18, in the southeastern part of North Carolina near Wilmington.
State and congressional boundaries will be reworked after the 2010 census, potentially creating another series of political and legal disputes in areas with sizable minority populations.
The case is Bartlett v. Strickland (07-689).