This from Bernie Grofman and Keith Gaddie (with the great Josh Rosenkranz as counsel of record) may be the most important brief filed in the Gill case case:

Amici seek to assist the Court in understanding recent developments in social science methodologies for identifying and measuring the extent of partisan gerrymanders. They do not take a position on whether, given the particular facts and expert witness analysis, the district court correctly decided this case. But amici firmly believe that partisan gerrymanders are justiciable, and that this Court should adopt an articulable standard for adjudicating partisan gerrymandering claims. Social science tools now allow courts to diagnose partisan gerrymanders with accuracy and precision. They also allow courts to distinguish ordinary, acceptable politicking from conduct that rises to the level of unconstitutional discrimination against voters based on their political views. If the Court again declines to adopt a standard for unconstitutional partisan gerrymandering, politicians will have free rein to violate associational and representational rights….

Modern, computer-driven redistricting now allows the political party in power to craft extremely sophisticated partisan gerrymanders. With vastly improved computer speed, memory, and storage, map drawers can design district lines so precisely that they simultaneously maximize their party’s gains and eliminate most competitive districts—ensuring that the party in power enjoys an electoral advantage that endures throughout the following decade, irrespective of voters’ subsequent choices.

Left unchecked, partisan gerrymandering fundamentally undermines our democracy. It is a basic tenet of fair elections that the parties must play by the same rules. But a partisan gerrymander violates that core principle: Under a successful partisan gerrymander, one party needs fewer votes to win representation than the other party. A partisan gerrymander places unequal burdens on voters’ opportunity to elect their representatives, based on the party with which they associate. And where the partisan gerrymander is unresponsive to electoral shifts, only the courts can provide a remedy.

This Court should hold that partisan gerrymandering claims are justiciable. To be precise, partisan gerrymandering occurs when a districting plan penalizes the minority in its ability to translate its voting support into seats compared to what might be expected from a plan drawn on the basis of neutral principles. But not all partisan gerrymanders are unconstitutional. The Court should adopt a test for unconstitutional partisan gerrymandering that requires a showing of three specific elements: partisan asymmetry, lack of responsiveness, and causation.

The first element, partisan asymmetry, is based on the idea that a citizen’s representational rights must not depend on the party with which he chooses to affiliate. Unlike a claim that the plaintiff is entitled to a specified number of seats, an asymmetry standard requires only that the parties and their supporters receive equal treatment—that they have like opportunity to translate their votes into representation. Thus, if Party A would garner, say, 60% of the seats when it wins 53% of the votes, Party B should also garner about 60% of the seats when it wins 53% of the votes. If it would not, partisan asymmetry is present. The second element, lack of responsiveness, screens out cases where the political process can provide a remedy. It examines whether a map is responsive to shifts in voters’ allegiances, such that any disparate effect on voters is unlikely to persist throughout the decade following redistricting. If a map is responsive, then when voters change their allegiances, their representation also changes, making judicial intervention unnecessary. If a map is not responsive, the courts may step in. The third element, causation, requires that, to be actionable, a disparate effect on voters must be the result of invidious, intentional discrimination against disfavored voters—and not merely the natural byproduct of ordinary districting practices or chance.4

These three elements are derived from the Court’s Equal Protection and First Amendment jurisprudence, and the social sciences offer tools for measuring each. Because each of these elements reflects a different concept, it is important to recognize that no one number tells it all. Rather, the Court should adopt a test for partisan gerrymandering that makes each of these three elements a necessary, but not sufficient, condition of a claim. In none of the Court’s prior partisan gerrymandering cases did the plaintiffs propose such a test, much less offer evidence of all three elements. And the statistical tools for detecting and measuring partisan gerrymanders have improved greatly since the Court last considered partisan gerrymandering in LULAC. Courts—assisted by competent experts—can now reliably and accurately identify and measure the impact of partisan gerrymanders, including determining whether invidious discrimination is the cause of any disparate burden on one political party, or whether any disadvantage results instead from permissible, neutral factors or random chance. The courts can, and should, play a role in policing improper partisan gerrymanders.