Under Supreme Court precedent dating from the Warren Court era, all elected representation must be based on population. Thus a system of legislative representation by county would not stand constitutional scrutiny. The representation of states in the US Senate is an exception to this principle in that the Constitution expressly provides for two Senators per state without regard to population.
I knew it was from a court
decision going back to around
1964 or so but that is not something
that is not beyond the possibility of
changing at some point. The
SCOTUS should turn it back to
the states to choose status quo
or change it.