Which case declared that "one person, one vote" is a constitutional principle?

The case that established "one person, one vote" as a constitutional principle is Reynolds v. Sims. This landmark decision by the Supreme Court in 1964 addressed the issue of legislative representation in state legislatures. The Court ruled that state legislative districts must be apportioned based on population, ensuring that each individual's vote carries equal weight in elections.

The "one person, one vote" principle is rooted in the Equal Protection Clause of the Fourteenth Amendment, highlighting the importance of fair representation. This ruling aimed to prevent the undue influence of less populated districts over more populated ones and reinforced the democratic principle that each citizen's vote should have equal value.

The other cases listed pertain to different constitutional issues: Brown v. Board of Education focused on racial segregation in public schools, Marbury v. Madison dealt with judicial review, and Roe v. Wade involved the right to privacy concerning abortion. Each of these landmark cases significantly impacted constitutional law but did not address the principle of "one person, one vote," which is specifically encapsulated in Reynolds v. Sims.