They said the results under the legislature's map were "remarkably similar" to election outcomes under earlier, court-drawn districts.
The issue is reaching the high court at a time when both Republicans and Democrats have improved the art of drawing congressional and legislative maps to entrench themselves in office for a decade at a time. The case dates back to a 2015 lawsuit when Democrats sued the state over legislative maps it drew in 2011.
The court has already, recently struck down racial gerrymandering in North Carolina, where lawmakers are required to draw new maps before holding another election.
The U.S. Supreme Court has never struck down a state redistricting plan on the basis of a partisan gerrymander.
Overturning these districting plans would create considerable instability as state legislators in various states - and most likely lower courts, too - would be charged with taking the step of drafting new plans, and these plans would have ripple effects as would-be and actual candidates reassess their campaign strategies under altered district boundaries.
But the justices have not thrown out state electoral maps drawn simply to give one party an advantage over another. The state is one of several battlegrounds where Republicans and Democrats fought to a virtual draw in last year's presidential election, but where Republicans enjoy election districts that have given them a almost 2-to-1 advantage in the state Assembly.
At issue is whether Republican lawmakers in Wisconsin drew legislative districts that favored their party and were so out of whack with the state's political breakdown that they violated the constitutional rights of Democratic voters. The map, Judge Kenneth F. Ripple wrote for the majority, "was created to make it more hard for Democrats, compared to Republicans, to translate their votes into seats". He says Democrats proved in court that their rights were violated and "now this story will be told on a national stage".
The Supreme Court had discussed the case at its June 8 and June 15 conferences, according to the court's docket on the case.
"Wisconsin's gerrymander was one of the most aggressive of the decade, locking in a large and implausibly stable majority for Republicans in what is otherwise a battleground state", said redistricting expert Thomas Wolf of the Brennan Center.
The U.S. Supreme Court said Monday that it will decide in its next term a case brought by Democratic voters in Wisconsin who argue that state Assembly districts are unconstitutionally gerrymandered to favor Republicans.
"This will be the biggest and most important election law case in decades". The court in a separate order delayed the drawing of new state Assembly district boundaries. That work is proceeding.
Democrats hope a favorable decision will help them cut into Republican electoral majorities.
Rick Hasen, a University of California-Irving law professor who blogs about election law cases, wrote Sunday night that the court granting a stay would indicate a good possibility that the court is positioned to reverse the lower court panel's decision in the case.