This article appears in the Fall 2017 issue of The American Prospect magazine. Subscribe here.

Gerrymandering is an old phenomenon with new dimensions and heightened significance for American democracy. Thanks to technology and political polarization, the effects of partisan gerrymandering since 2012 have been more pronounced than at any point in the previous 50 years. Close to a hundred congressional seats and thousands of state legislative seats have been strategically drawn to be noncompetitive at the expense of all other interests. As a consequence, tens of millions of voters have had no meaningful say in who represents them.

In this year's Supreme Court term, the justices have taken up the constitutionality of partisan gerrymandering in a case involving state legislative districts drawn by Republicans in Wisconsin. A key question for the Court will be whether neutral statistical tools can reliably detect partisan gerrymanders. We believe that such tools are available, and it is time for the courts to apply them.

Any party in power, if left unchecked, is likely to abuse the redistricting process. But the recent abuses come almost entirely from one party in particular. In the last redistricting cycle after the 2010 census, Republicans dramatically increased their representation through creative map-drawing in many states at once. Their efforts led to nationwide gains in representation larger than any in recent history.

While legal scholars tend to approach gerrymanders on a case-by-case basis, we have developed statistical tests to measure the extent of partisan bias whenever districts are redrawn. By applying those tests to all cases of congressional redistricting over the past half-century, we have been able to paint a unified picture of the trends in partisan gerrymandering. Available at gerrymander.princeton.edu, our data show that only since 2000 has extreme partisan distortion in congressional districting occurred in more than one state at a time. Those extreme distortions culminated in the Great Republican Gerrymander of 2012, encompassing seven states at once.

We define an extreme partisan gerrymander as a redistricting plan that simultaneously violates two or more statistical tests (see sidebar, “Can Math Assist in Saving Democracy?”). The use of multiple tests can help ensure the robustness of a judgment that a redistricting map is indeed a partisan gerrymander.

Although Democrats also gerrymander, Democratic gerrymanders that distort representation by more than one seat have been rare in the past half-century. The most prominent case occurred in California after the 1980 census, when a plan concocted by Representative Phil Burton netted Democrats three to five seats. Democrats devised similar schemes on a smaller scale in Texas in the 1990s and North Carolina in the 2000s. But at a national level, the total effect of these schemes was small.

Gerrymandering increased in frequency in 2002, with five state gerrymanders—four Republican, one Democratic—and again in 2012 to seven, this time all Republican (see figure). According to our analysis as well as that of the Brennan Center, Republicans have gained between 15 and 20 congressional seats above what partisan-neutral maps would have yielded.

Some analysts claim that Democrats have been at a disadvantage not because of redistricting but because their voters are more clustered geographically. But that geographic pattern does not fully explain Democrats' recent electoral disadvantage. On the basis of clustering alone, Democrats need to win the national popular vote for the House of Representatives by two percentage points to have an even chance of winning a majority of the seats. But since 2012, gerrymandering has increased the necessary national margin for Democrats to about eight percentage points. In individual gerrymandered states such as North Carolina or Pennsylvania, Democrats need to win by 15 percentage points or more to have a shot at taking a majority.

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Perhaps counterintuitively, individual legislators in gerrymandered districts appear not to be more polarized than others in their party. But gerrymandering still contributes to the partisan divide, simply by creating more districts in which the only election that matters is the primary. In places where the general election's outcome is a foregone conclusion, the most loyal partisan voters have great influence over who is nominated and therefore elected in November. This is true in any district that is dominated by one party, and gerrymandering increases the number of such districts.

Motive, Means, and Opportunity

What led to this sea change in partisan gerrymandering? Recent Republican maneuvers required the convergence of multiple factors, several of which were unavailable to Democrats. Like a crime procedural, the causes of modern partisan gerrymandering can be sorted into three parts: motive, means, and opportunity.

Motive: Partisan polarization. Extreme partisan redistricting is part of a general pattern since the mid-1990s that the legal scholar Mark Tushnet calls “constitutional hardball.” When politicians and parties play hardball, they change government in fundamental ways and in violation of customary norms, but without transgressing the letter of legal rules. Recent examples include the blockade of judicial nominees like Merrick Garland on party-line votes, abuse of Senate procedures to slow down business, and mid-cycle redistricting for partisan gain. In all these cases, partisans have overridden longstanding traditions of governance that were not mandated by law.

Traditionally, gerrymandering mostly protected individual incumbents of both parties, but in recent years it has increasingly been driven by partisanship. Since the Watergate era, the major parties have become ever more divided by ideology and policy preferences, and party loyalty has increased among both voters and legislators. Consequently, it makes more sense than ever for parties to invest in redistricting efforts to give themselves as many reliable seats as possible. In addition, control of the House of Representatives since 1994 has often hinged on very small seat margins. Under such circumstances it pays to draw districts not just to protect individual incumbents, but to enlarge the team.

Means: Voter clustering and redistricting technology.

With the benefit of new technology, gerrymanders have become considerably more sophisticated and durable.

Voters today increasingly cluster in residential communities that lean politically to the left or the right, as described in Bill Bishop's book The Big Sort and supported by later studies. The resulting ideological enclaves provide the raw material that redistricters use to draw predictably partisan districts. The basic strategy is to pack the opposing party’s voters into a few districts while distributing one’s own voters across many districts to win more seats by smaller but still secure margins. Modern computing technology and data science have made it possible to calibrate the size of these wins with great precision.

Redistricting can magnify the effects of natural population clustering enormously. The results are especially clear in cases where the party in control of redistricting changes from one redistricting cycle to the next. The congressional map in North Carolina, for example, went from a Democratic advantage in 2010 to an even larger Republican advantage in 2012. In 2010, a Republican wave year, Democrats won only 46 percent of the statewide vote yet took seven out of thirteen congressional seats. In 2012, after Republicans redrew the map, Democrats increased their vote share to 51 percent, yet won only four congressional seats.

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New technology has helped partisan redistricters maximize the seats they can win without winning more votes. Although computerized redistricting software has been in use since the 1990s, it truly came into its own after 2000. New software makes it feasible to draw complex maps and to check instantly the effect of moving a handful of precincts between districts. Thousands, millions, and even billions of possibilities can be explored with ease. Combined with analytics capable of predicting how most citizens will vote, the power of technology will enable even more sophisticated gerrymanders in 2020 and beyond.

Opportunity: The rise of single-party control. Compared with lawmakers in most other established democracies, state legislators in the United States have an unusual amount of control over electoral maps. In many of the 50 states, redistricting maps are enacted by the legislature and signed by the governor, just like any other legislation. If a party controls both legislative chambers as well as the governor’s mansion, there’s little to stop a determined majority from drawing the most advantageous maps possible.

In 2010, Republicans achieved unified control of 18 state governments. Those victories did not simply reflect the general gains the party made that year. In the REDMAP project, Republicans had directed money at state-level races in an attempt to ensure Republican majorities in closely contested legislatures. The net effect was one-party control over redistricting of 213 congressional seats and hundreds of legislative seats in large purple states such as Michigan, Ohio, Pennsylvania, North Carolina, and Virginia.

Democrats have played the gerrymandering game too, but in recent redistricting cycles they have not been in a position to achieve significant gains. In Maryland, they redrew the congressional district lines to unseat one of the state's two Republican representatives. A formerly Republican district that extends to Pennsylvania and West Virginia was engineered to extend tendrils to the outskirts of suburban Washington, D.C., pulling in enough Democratic voters to flip the district. While undesirable, this partisan maneuvering is within the bounds of pre-2000 partisanship and does not qualify as the extreme multi-seat distortion of the vote detected by our tests.

The Remedy: Courts and Reforms

As the 2020 redistricting cycle approaches, technology will only improve and partisanship shows little sign of abating. Now that the Republican Party has let the gerrymandering genie out of the bottle, who's going to put it back in?

Reform can take two routes. Federal courts could set a national standard that places guardrails to limit the most extreme offenses. And individual states may adopt new laws, either through voter initiatives or legislation, to limit or eliminate the ability of legislators of one party to create unfair maps. In this way, national standards and reformed state districting methods together may limit partisan gerrymanders.

The Supreme Court has an opportunity to act this October when it hears Gill v. Whitford, concerning legislative districts in Wisconsin. Of all the 2012 congressional and state gerrymanders, the Wisconsin Assembly map may be the worst. The trial court ruled the state’s districting scheme unconstitutional and ordered the map redrawn on the basis of extensive evidence of intent to create partisan advantage and overwhelming statistical evidence that the districting map did exactly that. Now the Supreme Court will decide whether the ruling will stand, with Justice Anthony Kennedy likely to be the key vote determining the outcome.

The questions in Whitford are rooted in three decades of indecision by the Supreme Court. Since 1986, the Court has struggled to develop a doctrine to limit partisan gerrymandering. In the 2004 case of Vieth v. Jubelirer, the Court splintered, with justices writing five separate opinions. Four justices voted to disallow partisan gerrymandering claims altogether on the ground that the cases raise a “political question” that the courts should not attempt to adjudicate. Four justices thought that courts could properly intervene but were divided as to the appropriate test for partisan gerrymandering. Justice Kennedy took a different tack. While agreeing that a suitable standard for judgment had not yet been developed, he left the door open for “clear, manageable, and politically neutral standards” to be developed that the Court could adopt in the future.

This time, academics and reform-minded lawyers have unified around a concept of fairness known as partisan symmetry.

Partisan symmetry is the idea that parties should be treated equally and should have roughly the same opportunity to translate votes into seats.

For instance, a map would be perfectly symmetrical if a party that wins 60 percent of the average district vote receives 80 percent of the seats so long as the opposing party would also have won 80 percent of the seats if it had earned 60 percent of the vote. Several simple standards to examine the fairness of maps have been floated, including Student's t-test for lopsided wins and the efficiency gap (see sidebar).

If the Court settles on a partisan symmetry standard and allows challenges to other maps to proceed, lower courts will do what they always do in the wake of a Supreme Court decision: identify and develop appropriate tests through the course of litigation. The first opportunities are already in the pipeline, and their outcome depends on Whitford. Common Cause and the League of Women Voters are challenging congressional maps in North Carolina, while Republican voters are challenging the Maryland gerrymander. Depending on the outcome of Whitford, gerrymanderers may survive or be brought to heel with a new, manageable standard.

The Next Front in the Battle

Even if the Court upholds Whitford and strikes down the Wisconsin gerrymander, its decision alone will not resolve the problem of partisan gerrymandering. The courts often enter the fray only after an election, and the litigation can take years. For example, the North Carolina state legislative map enacted for 2012 was ruled an unconstitutional racial gerrymander in 2016, a decision that the Supreme Court affirmed in May of this year. There is still no guarantee that the new maps will be fair when they are eventually drawn: The North Carolina legislature has made moves to replace its racial gerrymander with a partisan gerrymander ahead of the 2018 midterms.

Real protection against partisan gerrymanders can come only from preventive measures, enacted on a state-by-state basis. Such reforms can ensure statewide partisan symmetry and constrain how individual districts are drawn.

All reforms, however, are not created equal. For example, while reformers often seek to pass laws governing how the lines are drawn, those guidelines fail to constrain partisan malfeasance. A prime example of how such guidelines go awry is the battle over district shape. Critics sometimes hold up a rogue's gallery of maps, showcasing the most twisted districts drawn for partisan gain, as intuitive proof of bias. Compactness-based complaints, however, often fail in court.

Weird district shapes are good for a laugh, but they distract from real harms and can lead activists down the wrong path. A tortuous boundary does not always imply a partisan offense. Sometimes ugly, noncompact districts are the only way to satisfy more stringent criteria concerning equal population and race. Conversely, relatively straight and pretty boundaries can conceal a partisan offense. A straight boundary can cabin off a politically homogenous community, effectively packing it, or it can split the community in two, diluting the community's influence. Michigan is classified as a gerrymander by many metrics, yet its districts are reasonably compact. In 2016, when a court ordered Republicans in North Carolina to redraw the state's congressional map on racial grounds, the GOP came up with a visually less-offensive districting scheme that maintained its 10-3 advantage in the state congressional delegation. Judging the goodness of a district purely by the compactness of its shape is a legal dead end.

Corey Lowenstein/The News & Observer, File via AP Two North Carolina Republican state senators, Dan Soucek, left, and Brent Jackson, right, review historical maps of legislative districts, February 16, 2016. Republican-controlled redistricting in that state exemplifies the rise of extreme partisan gerrymanders.

Other guidelines on districting lines face similar challenges. Some states already mandate that mapmakers keep counties together, join “communities of interest,” and prohibit undue consideration of partisanship. But what is a community of interest? Maryland is attempting to perpetuate its 2012 gerrymander by calling the I-270 corridor a community of interest. And what happens when criteria conflict? Legislatures can easily choose between maintaining a county line or keeping a community of interest together, depending on which option suits partisan needs. With such a confused set of instructions, the addition of a partisan-symmetry standard would be more direct, logical, and reliable.

A more effective reform would remove legislators from redistricting. The independent commissions in California, Arizona, and other states have removed the fox from the henhouse, putting the map-drawing process into the hands of the general public. In California, anyone who wishes to join the commission may sign up on a website. After screening for conflicts of interest (for example, being married to a state legislator), five Democrats, five Republicans, and four independents are drawn from the pool of qualified applicants. These 14 citizens are responsible for drawing the maps, and in order for the final map to be adopted, three commissioners from each of the three voting blocs must approve it.

Independent commissions can achieve reforms well beyond the avoidance of partisanship. For example, they are often prohibited from drawing lines to protect incumbents, and commission-drawn maps typically have multiple competitive swing districts. Commissions have usually been established by ballot initiative, since parties with legislative majorities rarely vote to give up their power.

But since only 26 of the 50 states allow ballot initiatives, what about the remaining 24 states?

There is now some bipartisan support to curb gerrymandering.

In addition to many Democrats who take that position, the Republicans in favor of reform include Arnold Schwarzenegger, John McCain, Richard Lugar, John Danforth, Alan Simpson, John Kasich, Bob Dole, and other current and former congresspersons and state legislators who have filed pro-reform amicus briefs in the Wisconsin case. This kind of support at least opens the possibility that public pressure may induce legislators to establish independent redistricting commissions.

The Erosion of Governance

Partisan gerrymandering is part of a larger pattern of decades of political disruption on the national stage—the whole pattern of constitutional hardball we referred to earlier. Of all the hardball tactics, partisan gerrymandering is unusually powerful because it leverages state-level victories into national structural legislative advantage. Operatives employed by the Republican national party spent tens of millions of dollars on previously sleepy state legislative races in 2010, and the national branches of both parties are expected to spend much more in 2020. Gerrymandering represents part of a concerted effort for a highly organized faction to obtain a set of policy outcomes no matter what, without even needing to win a majority of votes at the ballot box.

Gerrymandering also separates politicians from their voters. Politicians are naturally interested in political survival, whether it be the next primary or general election. Safe districts make things easy. But in close districts, lawmakers are at perpetual risk of being caught on the wrong side of a divisive ideological issue. As swing districts are drawn out of existence, ever-larger numbers of representatives do not have to worry about electoral consequences when they cast a legislative vote, no matter how unpopular.

Gerrymandering is not just a federal problem. Thousands of American communities use districting for a variety of purposes such as public schools, city council elections, and utility districts. No matter how the Court rules in Whitford, some of the next battles in partisan gerrymandering will be fought at the local level.

American democracy relies on the faith of the American people. When legislators choose their voters and not the other way around, that faith erodes. Despite being a hard-to-understand offense, gerrymandering is nonetheless a major threat to representative democracy. Gerrymandering contributes to many voters' sense that government doesn't represent them. Until we limit the ability of incumbent parties to use gerrymanders to keep themselves in power, those voters will be right.