The founder of the idea of "checks and balances" themselves (Montesquieu, Secondat) said it, the Founding Fathers who were influenced by the two (Madison) wanted to charter it (Federalist n. 10), and Hobbes argued for it in Leviathan. The writing has been on the wall for CENTURIES to include The People as its own entity within the ontological power-struggle of American politics - we were just led to forget our own history.
To be honest I don't think that in these past ~250 years we've made that big of a progress in answering those two fundamental questions. I mean, the US Constitution starts with the very rousseau-ist "We the People of the United States", but a major part of the population living in said United States wasn't considered (fully) part of said People until the mid-19th century, some say that even now that process is an ongoing project.
