“One man, one vote” is not part of the Constitution.
It’s in the living and breathing rat constitution that supersedes our Constitution. It is why the shape of congressional districts is still a cage fight in many states some eight years after the last census, and why several states face bankruptcy.
It is now.
In Reynolds v. Sims, the Warren Court interpreted the 14th Amendment, in a decision that one justice labeled as "grotesque," to apply to all bodies not exempted by the Constitution itself. We know it as "one man/one vote." It is now a part of case law and is cited in precedents, such as in this case.
The Senate is exempt from Reynolds, and even Article V forbids the end of equal representation in the Senate unless states deprived of it are willing to ratify such a change. That would mean unanimous consent.
The Electoral College is exempt from Reynolds because the state legislatures have a plenary and non-justiciable right to appoint electors.