One-person, one-vote is a legal rule that one person’s voting power ought to be roughly equivalent to another person’s within the same state.
The rule comes up in the context of states gerrymandering and strategically drafting voting laws to increase the voting power of particular groups to the disadvantage of other groups. Equal protection, as guaranteed by the Fifth and Fourteenth Amendments, requires that each person be treated equally in their voting power, but precisely what that equality means, comes from a series of opinions issued in cases brought before the Supreme Court of the United States. The most relevant Supreme Court case is Reynolds v. Sims, 377 U.S. 533 (1964) (for the Wex entry, see Reynolds v. Sims (1964)). In Reynolds, the Court held that states must redistrict in a way that preserves state legislative districts with roughly equal populations, explaining, "The Equal Protection Clause requires substantially equal legislative representation for all citizens in a State regardless of where they reside." More recently, in Evenwel v. Abbott, 578 U.S. 54 (2016), the Supreme Court held that state legislatures may use total populations, as opposed to only voting-eligible populations, to determine demographics of districts and demonstrate compliance with the law.
For more on the one-person, one-vote rule, see this University of Florida Law Review article, this University of Michigan Law Review article, and this article in The Atlantic.
[Last updated in September of 2024 by the Wex Definitions Team]