In extended remarks, Chief Justice John G. Roberts Jr. expressed worry that the court’s authority and legitimacy would be harmed were it to start striking down voting districts in favor of one political party or another.
“That is going to cause very serious harm to the status and integrity of the decisions of this court,” he said.
Paul M. Smith, a lawyer for Democratic voters challenging a voting map in Wisconsin, urged the court to act. “You are the only institution in the United States that can solve this problem,” he told the justices.
The member of the court who probably holds the crucial vote, Justice Anthony M. Kennedy, asked questions suggesting that he thought the Supreme Court has a role to play in limiting partisan gerrymandering.
The Supreme Court has never struck down an election map on the ground that it was drawn to make sure one political party wins an outsize number of seats. The court has, however, left open the possibility that some kinds of political gamesmanship in redistricting may be too extreme.
The problem, Justice Kennedy wrote in a 2004 concurrence, is that no one has devised “a workable standard” to decide when the political gerrymandering has crossed a constitutional line.
On Tuesday, lawyers for the state of Wisconsin urged the justices to reject such a challenge to that state’s redistricting map, drawn by the Republican-controlled government, saying that Democratic critics were relying on flimsy and hypothetical social science evidence to prove that the maps led to the unconstitutional advantage of one party over the other.
Misha Tseytlin, the state’s solicitor general, warned that rejecting Wisconsin’s maps would spark “a redistricting revolution on these social science metrics” and would “shift districting from elected public officials to the courts.”
But during sharp questioning of the state’s lawyers, several justices hinted at the high stakes for democracy if parties are permitted to use sophisticated technology to devise voting maps that give them a significant, long-term advantage over their rivals.
Justice Elena Kagan said there is “good evidence” that the maps drawn by the Republicans in Wisconsin were designed to have “a certain kind of an effect, which was to entrench a party in power.”
Justice Kagan also pressed the state’s lawyers to explain their criticism of the social scientists who have concluded that the maps are overly partisan. She noted that lawmakers use the same technology and social science to draw the maps in the first place.
“So, too, those same technologies can be used to evaluate what they are doing,” Justice Kagan said.
The case, Gill v. Whitford, No. 16-1161, started when Republicans gained complete control of Wisconsin’s government in 2010 for the first time in more than 40 years. It was a redistricting year, and lawmakers promptly drew a map for the State Assembly that helped Republicans convert very close statewide vote totals into lopsided legislative majorities.
In 2012, after the redistricting, Republicans won 48.6 percent of the statewide vote for Assembly candidates but captured 60 of the Assembly’s 99 seats.
Democratic voters sued, saying the maps violated the Constitution.
The case is part of a larger debate over politics in redistricting, one that has taken on new urgency with the advent of sophisticated software. Both parties have engaged in partisan gerrymandering, but these days, Republicans have an advantage following a wave of victories in state legislatures that allowed lawmakers to draw election maps favoring their party.
Some critics, like Arnold Schwarzenegger, a Republican and the former governor of California, say districts should be drawn by independent commissions rather than politicians. Prominent Democrats, including former President Barack Obama and his attorney general, Eric H. Holder Jr., are pushing to undo the redistricting gains Republicans made after the 2010 census when the next census is taken three years from now.
Last year, a divided three-judge Federal District Court panel ruled that Republicans in Wisconsin had gone too far. The map, Judge Kenneth F. Ripple wrote for the majority, “was designed to make it more difficult for Democrats, compared to Republicans, to translate their votes into seats.”
The decision was the first from a federal court in more than 30 years to reject a voting map as partisan gerrymandering.
Wisconsin officials say that the lopsided representation of Republicans in the State Legislature is a product of geography rather than gerrymandering. Democrats have packed themselves into cities, effectively diluting their voting power, while Republicans are more evenly distributed across most states, the brief said.
Judge Ripple acknowledged that the distribution of the population explains at least some part of the gap.
“Wisconsin’s political geography, particularly the high concentration of Democratic voters in urban centers like Milwaukee and Madison, affords the Republican Party a natural, but modest, advantage in the districting process,” he wrote.
But he added that partisan gerrymandering amplified that advantage.