WASHINGTON (Reuters) - It is a political practice nearly as old as the United States - manipulating the boundaries of legislative districts to help one party tighten its grip on power in a move called partisan gerrymandering - and one the Supreme Court has never curbed.

FILE PHOTO: The U.S. Supreme Court building is pictured in Washington, DC, U.S. on June 26, 2017. REUTERS/Yuri Gripas/File Photo

That could soon change, with the nine justices making the legal fight over Republican-drawn electoral maps in Wisconsin one of the first cases they hear during their 2017-2018 term that begins next month. Their ruling in the case could influence American politics for decades.

Wisconsin officials point to the difficulty of having courts craft a workable standard for when partisan gerrymandering violates constitutional protections. Opponents of the practice said limits are urgently needed, noting that sophisticated technological tools now enable a dominant party to devise with new precision state electoral maps that marginalize large swathes of voters in legislative elections.

“There is a sense that something has gone amiss with American democracy, that there is this effort to rig the rules of the game,” said Michael Li, an expert in redistricting at New York University School of Law’s Brennan Center for Justice. “Gerrymandering used to be a dark art, and now it’s a dark science.”

The justices will hear arguments on Oct. 3 in Wisconsin’s appeal of a lower court ruling that found that the electoral map drawn by state Republicans ran afoul of the U.S. Constitution.

The map, drawn after the 2010 U.S. census, enabled them to win a sizable majority of Wisconsin legislative seats despite losing the popular vote statewide to the Democrats. The party’s majority has widened since.

The justices must decide whether courts should have a say in such matters. The Supreme Court for decades has been willing to invalidate state electoral maps on the grounds of racial discrimination but never those drawn simply to give one party an advantage.

The justices have never settled on a standard by which partisan gerrymandering claims can be measured. In a 2004 case, Justice Anthony Kennedy left the door open for a “workable standard” to eventually be found.

Kennedy, a conservative who sometimes sides with the liberal justices on a court with a 5-4 conservative majority, could cast the deciding vote in the Wisconsin case.

A federal three-judge panel ruled 2-1 last November that Wisconsin’s redistricting plan violated the U.S. Constitution’s 14th Amendment guarantee of equal protection under the law and First Amendment right to freedom of expression and association.

Over the decades, both Republicans and Democrats have been accused of gerrymandering. Since 2010, Republicans’ control of redistricting has coincided with major seat advantages for them in state legislatures, the Brennan Center said.

It is not just Republicans who are accused of abuses. Republican voters sued over districts drawn by Democratic lawmakers in Maryland and have appealed to the Supreme Court.

State and federal legislative district boundaries are reconfigured after the U.S. government conducts a census every decade so that each one contains about same number of people, typically by the party that controls the state legislature.

The Republican National Committee and several conservative groups have backed Wisconsin, but leading Republicans including Senator John McCain, 1996 presidential nominee Bob Dole and Ohio Governor John Kasich have joined critics who argue that partisan gerrymandering distorts the democratic process.

‘SOCIAL-SCIENCE HODGEPODGE’

Wisconsin Republicans argued that election results since their redistricting plan was passed in 2011 reflect the state’s political geography, with Democrats concentrated in cities like Milwaukee and Madison and Republicans more spread out around the state.

The state also took issue with metrics that the lower court used to determine that there was a significant partisan bias in the redistricting plan. This “social-science hodgepodge,” the state told the justices, makes it impossible for judges to fairly determine when an electoral map is unlawful.

“Plaintiffs’ social-science approach would sow chaos. Each legislatively drawn plan would be immediately challenged in federal court,” the state said in a legal brief to the justices.

Nicholas Goedert, a Virginia Tech redistricting expert who testified in court for Wisconsin, said the metrics may be inappropriate because their results can change from one election to the next. Goedert noted that large shifts in the mood of voters can cause even highly biased electoral maps to flip.

“Partisan maps have a tendency to backfire on the party that drew them,” Goedert said.

The case began in 2015 when a dozen Wisconsin Democratic Party voters sued state election officials claiming the redistricting law intended to discriminate against them for their political beliefs and create enduring Republican majorities.

They urged the justices to either greenlight the lower court’s method of deciding cases of partisan gerrymandering, or create their own.

“Government should treat voters equally regardless of their viewpoint and we have ways to measure it,” said Danielle Lang, an attorney for the plaintiffs.

If the justices rule that courts must stay out of this highly political process, voters will lose, Lang said.

“There would be no way for voters to rein in partisan gerrymandering, no way for voters to take back control of their government.”

(For a graphic on Wisconsin state electoral maps, click tmsnrt.rs/2haOPD5)