It is a common practice for state legislatures to redraw voting districts to confer an advantage on the governing party.
Paul Smith, a lawyer for the Campaign Legal Center, said gerrymandering "is worse now than any time in recent memory".
The Wisconsin case is one of several gerrymandering accusations to reach the court in recent months.
That was evident a year ago in North Carolina, when state lawmakers were ordered by a federal court to redraw the state's racially gerrymandered congressional districts.
A Supreme Court ruling faulting the Wisconsin redistricting plan could have far-reaching consequences for the redrawing of electoral districts due after the 2020 USA census. Let's say you have a strongly Democratic district.
Critics have argued the current legislative maps unfairly stack the deck for Republicans district by district, pointing to the 2016 presidential election results as an example. Federal judges have ruled the Texas Legislature intentionally diluted the voting power of African-American and Latino residents.
The justices regularly are called to invalidate state electoral maps that have been illegally drawn to reduce the influence of racial minorities by depressing the impact of their votes.
The justices previously had declared a June 30 deadline for adoption by the 2017 Legislature and Brownback of a plan created to correct constitutional problems in the state's approach to financing K-12 public education.
After winning control of the state legislature in 2010, Republicans redrew the statewide electoral map and approved the redistricting plan in 2011.
It was statistical measurement of how many votes in each party were wasted, either because they were so diluted in a district they could never achieve a majority or because they were so concentrated in a district that they were in excess of what was needed to get a majority.
After the 2011 redistricting in Wisconsin, Republican candidates won 60 of the 99 seats in the State Assembly. Or, they pack those areas into one congressional seat, conceding a seat but ensuring fewer voters of the opposing party are in districts they can win.
Enter the Wisconsin case.
"And they would argue that the new legal standard is such that the IL map doesn't measure up constitutionally and they would have a pretty good argument that it should be fixed before the next round of redistricting comes along", Smith said. The Republican National Committee and a dozen large Republican states have asked the court to reverse the Wisconsin decision.
In that case, Kennedy voted to uphold disputed congressional districts in Pennsylvania, saying the challengers hadn't provided a workable standard for determining whether partisanship played too big a role. I also argued that it was likely that the lower court's decision would be eventually overturned.
In March, the Supreme Court held the K-12 public education financing system wasn't reasonably calculated to have all Kansas public education students meet or exceed the minimum constitutional standards of adequacy. "Across the country, we're witnessing legislators of both parties seizing power from voters in order to advance their purely partisan purposes".
Gerrymandering has become more precise as computer software has become more sophisticated, enabling map-makers to divide counties, cities and even neighborhoods to maximize their political advantage.