Both plato and aristotle, but aristotle thought that any election-based state turned out in practice, to be an oligarchy or aristocracy, not a democracy (which he define as rule by the poor, with random selection by lot).

Aristotle's politics books 4-6 talk a lot about this:

http://classics.mit.edu/Aristotle/politics.4.four.html

In other words, what today we call "representative democracy", the ancient greeks correctly identified as oligarchy.