We have argued that the now well-developed statistical methodology used in the scholarly literature to measure partisan bias and electoral responsiveness can readily form the evidentiary basis for a metric that can be used to measure partisan gerrymandering effects,80 and it can allow courts to craft a “clear, manageable, and politically neutral” measure of “the particular burden a given partisan classification imposes on representational rights.”81 The level of partisan bias (which can be calculated as the difference between the seat shares that each of the two parties would receive if they each had received identical shares of the vote) can be used to evaluate the extent to which “each political group in a State [has] the same chance to elect representatives of its choice as any other political group.” 82
However, as we have emphasized earlier, while partisan bias can provide the basis of measuring the magnitude of inequality of treatment, the issue of when inequities rise to the level of a constitutional violation is a quite distinct question. In this section we focus on five potential approaches to craft a judicially manageable standard for unconstitutional partisan gerrymandering that build on the concept of partisan bias to identify legal thresholds for prima facie evidence of equal protection violations. Each of these approaches can be seen as straightforward adaptations of constitutional or statutory standards that the Supreme Court has adopted in other areas of voting rights case law. The latter two arise from ideas in Justice Kennedy’s opinion in LULAC.
The Supreme Court could adopt a rule that requires plans that create as little partisan bias as practicable. Such a rule could be adapted from the Supreme Court’s jurisprudence in the area of standards for acceptable deviations from population equality in the context of Congressional districting. In the one-person, one-vote context, the Supreme Court has required zero population deviation for Congressional districts, while still permitting the State to justify minor population deviances by showing a compelling need.83Moreover, the State need only achieve complete population equality “as nearly as practicable.”84 So long as a State attempts in good faith to create equal districts, the plan passes constitutional muster.85
If this were the approach taken by the Court, States would need to make a good-faith effort to achieve as little partisan bias as possible. States can comply with the “as little as possible” rule quite simply. The sophistication of computer mapping technologies allows States to evaluate partisan bias in creating a redistricting plan. States may then attempt to justify deviations from political symmetry by pointing to a legitimate interest such as compactness, respecting municipal boundaries, minority rights, or respecting communities of interest.86
Disqualify plans with partisan bias that deviate from symmetry by at least one seat
The Supreme Court could adopt a rule that a plan can be overturned as an unconstitutional partisan gerrymander only if it can be shown that the plan locks in political advantage for one political party of one or more legislative seats as compared to a plan that was symmetric with respect to the parties. Through the use of experts, a plaintiff would have to prove that a map would cause a group to lose at least one seat. Such a rule could be adapted from the Supreme Court’s jurisprudence in the area of standards for minority vote dilution in the context of Section 2 of the Voting Rights Act.87
Disqualify only those plans with egregious levels of partisan bias (defined in terms of a specified percentage point threshold)
The Supreme Court could adopt a rule setting a standard for egregious gerrymanders, by specifying a threshold level of partisan bias in percentage point terms such that plans which exhibit levels of bias below that threshold are regarded as prima facie constitutional, and which requires that plans with partisan bias above that threshold to be justified in terms of compelling, legitimate, and rational state policies and interests. Such a rule could be adapted from the Supreme Court’s jurisprudence in the area of standards for acceptable deviations from population equality in the context of state and local districting.88
In this third approach, if the average difference in the seat percentage between the two parties, given an empirically reasonable value of the average district vote, is above a particular threshold (whether it be 3, 5, or 10 percentage points), the plan would be prima facie unconstitutional. For example, suppose the court set the standard at 10 percentage points. In this situation, a plan would not be unconstitutional unless one party could be expected to capture 10 percentage points more seats than the other party even if the two parties received identical shares of the vote. 89 This unconstitutional level would be reached if the Democrats received 65% of the seats if they received 55% of the votes, but the Republican Party received 75% of the seats given the same 55% vote share.
Each of the three approaches above is based on a different but easy-to-manage judicial standard, and one that has been applied in other areas of the law. For example, since States know the total population before redistricting, and because application of the one-person, one-vote standard is straightforward, relatively little litigation results. Similarly, whenever a State redistricts and uses existing computer technology so that we can match (past) election outcomes to geographic units, measuring prospectively or retrospectively the partisan bias of a plan is also straightforward.90