Wiki has a good post about this:
during the height of related civil rights activities.
[5][6][7][8][a] Applying the
Equal Protection Clause of the United States Constitution, the
Supreme Court majority opinion (5-4) led by
Chief Justice Earl Warren in
Reynolds v. Sims (1964) ruled that state legislatures, unlike the
United States Congress, needed to have representation in both houses that was based on districts containing roughly equal populations,
You notice there are several good references to check out as well
Actually, I disagree with the Court on this - as the US Senate and the Electoral College do NOT follow the "One Man One Vote" The US Senate was purposely set up that way so that large population States/Commonwealths would not be able to "gang up" on the smaller ones. And likewise - States and Commonwealths with several large urban populations can now have majorities in State houses and have total control.