Series: Redistricting Opaque redistricting groups are being quietly bankrolled by corporations, unions and others to influence redistricting. They aim to help political allies — and in the process they're hurting voters.

In a recent interview, former Supreme Court Justice John Paul Stevens called Maryland's current redistricting plan, which draws districts to benefit the state's Democrats, "outrageously unconstitutional."

And that's not just because the Democratic plan divides a district with a majority of minority voters into several mostly white districts, an element of the plan that may face a legal challenge under the Voting Rights Act. It's because, in Stevens' judgment, drawing district lines to benefit one party violates the right of citizens to equal protection under the law.

"The government cannot gerrymander for the purpose of helping the majority party; the government should be redistricting for the purpose of creating appropriate legislative districts," Stevens said in a SCOTUSblog interview.

Of course, politicians have been drawing district lines for their own advantage since the days of the founding fathers, when Patrick Henry gerrymandered a Virginia district to try to keep James Madison out of Congress.

But is this kind of gerrymandering illegal?

While the Supreme Court has consistently found certain types of racial gerrymandering to be illegal, it has a much more ambiguous record on partisan gerrymanders in which voters are grouped or split based not on race but on their political orientation.

In his time on the Supreme Court, Stevens consistently opposed so-called partisan gerrymandering, but he was often in the minority.

While the Supreme Court ruled in 1986 that partisan gerrymandering was unconstitutional and could be challenged in court, it set such a high standard of proof that it made legal challenges of such districts extremely difficult. Since then, the Court has remained divided on whether there is any viable way to set a judicial standard for what makes a given district an illegal gerrymander.

Stevens has long argued that some kind of standard — based, to begin with, on the principles of compactness and contiguity — is possible.

"This is one of my major disappointments in my entire career: that I was so totally unsuccessful in persuading the Court on something so obviously correct," Stevens said.

As we explained in an investigation of special interests and redistricting, and in our latest music video, the ways that politicians manipulate districts are so well-known that political insiders have a special gerrymandering vocabulary: Politicians can "pack" certain communities into a single district, "bleach" out minorities, "crack" troublesome voting blocks between different districts, "kidnap" a troublesome representative by putting his or her house in a separate district from his or her former constituents, or "hijack" a district by redrawing the lines to pit two incumbents from the same party against each other. (For more details on these tactics, check out our Devil's Dictionary of Redistricting.)

While Stevens has long judged these kinds of tactics unconstitutional, other justices have been more skeptical. Justice Antonin Scalia has argued that the "fairness" of districts is not "a judicially manageable standard," and that there is no constitutionally discernable basis for deciding whether a district is an illegal partisan gerrymander.

It's worth noting that even Stevens' suggestion that courts base their evaluation of districts on the traditional principles of "compactness and contiguity" is problematic. While compact and contiguous districts may look good on a map, they aren't necessarily fair.

These seemingly "neutral" standards "really are incredibly arbitrary," said Keesha Gaskins, senior counsel at the Brennan Center for Justice, a nonpartisan group. The shape of a district may be easy to judge at a glance, but odd-looking districts may simply reflect the real shape of a community.

And judging districts to be "illegal gerrymanders" just because they're filled with voters who support one party would also be a mistake, Gaskins said. "It can be that communities of interest line up along partisan lines."

Despite the difficulty, Gaskins said, "we all want to see a workable standard under which these claims can really be adjudicated."

So far, this "workable standard" has proved elusive.

While the Supreme Court opened the door to partisan gerrymandering challenges in 1986, it set a high bar: Plaintiffs would have to prove that the shape of a district demonstrated "both intentional discrimination against an identifiable political group and an actual discriminatory effect on that group."

But this standard proved nearly impossible to meet in practice. As Whitney Eaton noted in a University of Richmond Law Review article:

The twenty partisan gerrymandering cases that followed Bandemer resulted in the federal courts denying relief in each and every one, leaving commentators to conclude that its "standards are fundamentally unworkable and incorporate such ambiguous and unclear commands as to be unfit for any manageable form of judicial application."

In a 2004 partisan gerrymandering case, the Court deadlocked on the question of whether partisan gerrymandering, given the difficulty of establishing a standard, should be subject to judicial review. Four justices argued yes — but proposed very different standards for how to tell whether a district has been gerrymandered. Four justices argued no, and one, Anthony Kennedy, argued that while no standard for judging what makes a district "fair" had emerged yet, it might be possible to find one.

In the Supreme Court's most recent consideration of partisan gerrymandering, in 2006, the confusion persisted.

While defining partisan gerrymandering has always been difficult, the Brennan Center's Gaskins said increasingly sophisticated mapping software makes the issue more problematic — and more pressing.

"It is a real challenge, as technology becomes more and more precise, whether or not these blunt legal responses are going to be able to adequately address these measures," she said. "Now you can buy data from any marketing company and draw districts that are highly protective or highly exclusive, and it changes the landscape in a way that has not been contemplated by the courts."

Gaskins said she expects to see the Court return to the question of redistricting in the next year.

"There does have to be a line where partisan gerrymandering goes too far," she said.