Thornburg v. Gingles, 478 U.S. 30 (1986), was a United States Supreme Court case in which a unanimous Court found that "the legacy of official discrimination ... acted in concert with the multimember districting scheme to impair the ability of "cohesive groups of black voters to participate equally in the political process and to elect candidates of their choice."

[4]: 60–61 [5] Congress responded by passing an amendment to the Voting Rights Act which President Ronald Reagan signed into law on June 29, 1982.

[10]: 708–709  An at-large election can dilute the votes cast by minority voters by allowing a cohesive majority group to win every legislative seat in the jurisdiction.

[17] In an opinion by Justice William J. Brennan joined partially by Justices Byron White, Thurgood Marshall, Harry Blackmun, and John Paul Stevens the Court used the term "vote dilution through submergence" to describe claims that a jurisdiction's use of an at-large/multimember election system or gerrymandered redistricting plan diluted minority votes, and it established a legal framework for assessing such claims under Section 2.

[21] As illustration Justice Brennan notes that 47.8% of the black population of Halifax County, North Carolina lives in poverty, compared with only 12.6% of whites.

Justice O'Connor, a former Arizona statehouse legislator, began by noting that Senator Bob Dole, "the architect of the compromise",[19]: 96  had insisted the 1982 amendment explicitly disclaim any right to racially proportional representation.

In Bartlett v. Strickland (2009),[23] the Supreme Court held that the first Gingles precondition can be satisfied only if a district can be drawn in which the minority group comprises a majority of voting-age citizens.

[18] The Court emphasized that the existence of the three Gingles preconditions may be insufficient to prove liability for vote dilution through submergence if other factors weigh against such a determination, especially in lawsuits challenging redistricting plans.

In particular, the Court held that even where the three Gingles preconditions are satisfied, a jurisdiction is unlikely to be liable for vote dilution if its redistricting plan contains a number of majority-minority districts that is proportional to the minority group's population.

[17] Three judge courts made up of all Democratic appointees have ruled in favor of Section 2 liability in 41% of cases, contrasted with 11% under the all Republican appointed panels.

In Shaw v. Reno (1993) the Supreme Court 5-4 struck down North Carolina's attempt to create two majority minority districts.

After hearing the case three more times, in Easley v. Cromartie (2001) the Supreme Court would 5-4 uphold the redistricting because the General Assembly's motivations had been purely political.