Thornburg v. Gingles establishes a three-part test for minority vote dilution

The Supreme Court unanimously adopted a three-factor framework — the Gingles preconditions — for determining when multimember or at-large election districts illegally dilute minority voting power under amended Section 2 of the Voting Rights Act. Justice William Brennan wrote for the Court, requiring plaintiffs to show the minority group is sufficiently large and geographically compact, politically cohesive, and faces majority bloc voting that usually defeats its preferred candidates. The case arose from North Carolina legislative districts and governed redistricting litigation for nearly four decades until Louisiana v. Callais.