It's called gerrymandering -- a term that arises from a district shaped like a salamander that was drawn during the 1810 term of Massachusetts Gov. Elbridge Gerry. Two hundred years later, legal experts are still divided on the racial and partisan considerations at issue.
Earlier this month, Justice Elena Kagan, writing for the majority of the Supreme Court, tore up two congressional district maps in North Carolina, holding that they amounted to an unconstitutional racial gerrymander. "A state may not use race as the predominant factor in drawing district lines," she wrote, referencing a 1993 court standard, "unless it has a compelling reason."
The ruling was a victory for Democrats and civil rights groups who had challenged the North Carolina maps arguing that they unnecessarily packed African-Americans into two districts. This made it easier for African-Americans to re-elect incumbents to those two seats, but diluted their votes in surrounding areas.
But it's the next step the Supreme Court takes that could really change the game. Here's why:
The Supreme Court has a standard limiting overreliance on race in map-drawing except under the most limited circumstances. But it has never been successful in developing a test concerning a much thornier issue: partisan gerrymandering.
"The court has said that too much partisanship is illegal," said Justin Levitt, a professor of law at Loyola Law School. "But it hasn't yet decided how much is too much."
But that could soon change, and Justice Anthony Kennedy's vote -- as it is on so many other issues -- could be key.
"For most Americans, it's obvious that our elected officials shouldn't be able to punish voters based on what party they prefer," said Levitt. "A Supreme Court decision setting limits on drawing districts for partisan advantage would substantially change the way that local, state, and congressional districts are drawn after the next census."
In most states, the maps are drawn after all by the party in power after each census, meaning neither party has a guarantee of controlling the districts indefinitely. But given US House districts generally survive for 10 years -- or five elections -- the impact on policy and government is substantial.
Levitt and others believe that the legislators in charge of drawing the maps have gone to new extremes impacting voters' right to fair representation under the First and Fourteenth Amendment.
In the coming days and weeks, the court will deal with two separate cases about partisan gerrymanders. The issue deeply divided them back in 2004 in a case called Vieth v. Jubelirer.
The conservatives on the court felt that the issue should be handled by the political branches. But Kennedy at the time was unwilling to bar all future claims of injury from partisan gerrymanders.
The court recognizes, for instance, that the map-drawing process is political, and that there will always be a certain amount of partisan politics involved. After all, it is politicians drawing political lines.
As Justice Antonin Scalia wrote in Vieth, "Political gerrymanders are not new to the American scene." He even noted there were allegations that Patrick Henry attempted, unsuccessfully, to gerrymander James Madison out of the First Congress.
But, Scalia concluded, "no judicially discernible and manageable standards for adjudicating political gerrymandering claims have emerged."
"The justices have struggled to figure out where to draw the line between acceptable partisan influence and an excessive influence that burdens the right to vote," said Danielle Lang of the Campaign Legal Center.
"Essentially, the court has not yet settled on a rule to determine the 'how much is too much' question," she said.
The 'efficiency gap'
Lang's group is behind one of the cases making its way to the court. She represents Wisconsin Democratic voters who are challenging district maps. Democrats claim that the maps discriminated against Democratic voters by diminishing the strength of their votes.
Last fall, a divided three-judge panel in Wisconsin held that the redistricting plan "was intended to burden the representational rights of Democratic voters throughout the decennial period by impeding their ability to translate their votes into legislative seats."
The court accepted the plaintiffs' standard based in part on the new work of political scientists who used voting data to calculate the amount of bias against one party or another in the maps.
The formula is called the Efficiency Gap.
"We proposed a standard that uses political science quantitative measures -- a new standard that has never been presented to the courts before," said Lang. "The court endorsed the use of those measures as evidence of the harmful effects of partisan gerrymandering."
Wisconsin state Attorney General Brad Schimel blasted the ruling and the standard. "Our maps are lawful and constitutional under any standard," he said in a statement.
Justices are also looking at another challenge to the North Carolina map that alleges an illegal partisan gerrymander.
A lower court denied a partisan gerrymander claim, but left the door open to future claims if plaintiffs did propose a standard. Supreme Court justices discussed the pending case behind closed doors last week.
Marc E. Elias, who served as the general counsel for Hillary Clinton's 2016 presidential campaign, is the lead lawyer in the case.
"Having been told last week that they can't engage in illegal racial gerrymandering, Republican legislators may now face a case in which they can't illegally use partisanship," Elias said in an interview.
"This will hopefully, finally, lead to truly fair redistricting throughout the country," he said.