Wisconsin is appealing a lower court's ruling that the electoral map devised by state Republicans had such extreme partisan aims that it violated the constitutional rights of voters.
The nine justices began questioning lawyers in the scheduled hour-long arguments in the case, one of the biggest they will decide in their new term that began on Monday and runs through June.
The court is considering the legality of partisan gerrymandering, the practice that began two centuries ago of manipulating boundaries of legislative districts to benefit one party and diminish another, and whether the Republicans who drew Wisconsin's electoral map intended to repress the clout of Democratic voters.
Electoral maps sometimes concentrate voters who tend to favor the minority party into a small number of districts to reduce their statewide voting power - called packing - and distribute the rest of those voters in other districts in numbers too small to be a majority - called cracking.
The Supreme Court, which currently has a 5-4 conservative majority, for decades has been willing to invalidate state electoral maps on the grounds of racial discrimination but never those drawn simply for partisan advantage.
The case gives the justices a chance to establish the first standards to gauge when electoral maps drawn to benefit one particular party violate the Constitution.
A federal three-judge panel ruled 2-1 last November that Wisconsin's redistricting plan violated the Constitution's guarantee of equal protection under the law and right to freedom of expression and association because of the extent to which it marginalized Democratic voters.
Challengers to the maps, a group of Democratic voters, said gerrymandering is getting more extreme due to increasingly precise voter data and mapmaking technology, leaving voters with no recourse but the courts. The Wisconsin Republicans argue that courts should have no say in disputes over partisan gerrymandering.
The Supreme Court's regular swing vote, Anthony Kennedy, could decide the case's outcome. In a 2004 ruling in another case, Kennedy parted with his conservative colleagues to suggest that if partisan gerrymandering goes too far, courts may have to step in, and that finding a "workable standard" might be possible.
State and federal legislative district boundaries are reconfigured after the U.S. government conducts a census every decade to make each one contain about same number of people. This redistricting typically is carried out by the party that controls the state legislature.
The Republican National Committee and several conservative groups have supported Wisconsin. Some prominent Republicans including current senator and 2008 presidential nominee John McCain, 1996 presidential nominee Bob Dole and Ohio Governor John Kasich have joined critics who argue that partisan gerrymandering can distort the democratic process.
Gerrymandering is named for Elbridge Gerry, a Massachusetts governor known for the practice in the 1810s.
(Reporting by Andrew Chung; Editing by Will Dunham)
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