By Andrew Chung and Lawrence Hurley
WASHINGTON (Reuters) - The U.S. Supreme Court on Monday dealt a setback to election reformers by declining to use high-profile cases from Wisconsin and Maryland to curb the ability of state lawmakers to draw electoral districts purely for partisan advantage.
Sidestepping major rulings, the nine justices decided both cases on narrow legal grounds and put off perhaps until their next term, which begins in October, a more definitive ruling on the contentious practice known as partisan gerrymandering.
In one decision, the court handed a victory to Wisconsin Republicans who drew state electoral districts that helped entrench their party in power by throwing out on a 9-0 vote a lower court ruling that the districts deprived Democratic voters of their constitutional rights including equal protection under the law.
The Supreme Court found that the Democratic voters who sued to block the Republican-drawn electoral map lacked legal standing to bring the case because they challenged it on a statewide basis rather than focusing on individual legislative districts.
The ruling makes it harder for plaintiffs to challenge maps statewide but does not close the door on such lawsuits. To do so from now on, voters in each district of a state will need to show that their own voting clout has been diluted.
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In the Maryland case, the court in an unsigned opinion decided not to immediately block a Democratic-drawn U.S. House of Representatives district challenged by Republican voters but allowed the case to proceed.
Election reformers in both parties had hoped the justices would rein in the intensified use of partisan gerrymandering, a practice in which the party that controls a state legislature uses the process of redrawing electoral districts after the U.S. census every decade to tighten its grip on power by diluting the influence of voters who tend to support the rival party.
Opponents have said partisan gerrymandering has begun to warp American democracy by muffling large segments of the electorate, violating voters' constitutional rights. Democrats in particular have accused Republicans of escalating partisan gerrymandering this decade, helping President Donald Trump's party maintain control of the U.S. House of Representatives and state legislatures.
Critics of gerrymandering have said it has become ever more extreme through the use of precise voter data and computerized modeling to devise electoral maps.
The justices could soon take up another case involving a challenge to Republican-drawn congressional districts in North Carolina that could give them another chance in their next term to issue a broader ruling on partisan gerrymandering.
Writing for the court in the Wisconsin case, Chief Justice John Roberts said electoral maps on a statewide basis cannot be challenged as a whole. "In this case the remedy that is proper and sufficient lies in the revision of the boundaries of the individual's own district," Roberts said.
Roberts said the court was leaving for another day whether judges have the power to remedy claims of unconstitutionally biased electoral maps. But he cautioned that plaintiffs must prove that their individual voting choices were harmed in their own districts.
"The Supreme Court missed an opportunity today to lay down a firm marker as to when partisan gerrymandering is so extreme that it violates the constitutional rights of voters. But the court permitted lawsuits against unfair maps to continue," said Dale Ho, a lawyer with the American Civil Liberties Union.
The justices sent the Wisconsin dispute back to a federal district court to give the Democratic voters another chance to prove they were harmed on a concrete basis. Conservative Justices Clarence Thomas and Neil Gorsuch said they would have dismissed the case outright.
Democratic Party voters in Wisconsin sued state election officials in 2015 claiming a Republican-drawn legislative map was intended to discriminate against them for their political beliefs and create enduring Republican majorities.
State Republicans defended their map as lawful, saying election results reflect the state's political geography, with Democrats concentrated in cities such as Milwaukee and Madison, and Republicans more spread out around the state.
The brief five-page ruling in the Maryland case was limited to rejecting the Republican voters' attempt to block the congressional district while litigation moves forward.
Republican voters sued Maryland after the legislature in 2011 redrew boundaries for the state's Sixth District in a way that removed Republican-leaning areas and added Democratic-leaning areas.
Redistricting in most states is done by the party in power, though some states in the interest of fairness assign the task to independent commissions.
(Reporting by Andrew Chung and Lawrence Hurley; Editing by Will Dunham)