The Supreme Court clearly established the one person, one vote principle for all legislative districting except the U.S. Senate.
Many states had state senates apportioned on a geographical basis, providing an equal number of senators to each county no matter how widely the population varied, just as the U.S. Senate was apportioned. The Supreme Court, by an 8-1 vote, ruled that both houses of a state legislature had to be apportioned according to the one person, one vote principle that the Court laid down in Gray v. Sanders
Baker v. Carr
Colegrove v. Green
Gray v. Sanders
Kirkpatrick v. Preisler
Mahan v. Howell
Reapportionment Cases
Representation, fairness of
Wesberry v. Sanders