The Greek historian Polybius (ca. 200 BCE) observed characteristic weaknesses of democracy, aristocracy, and monarchy. He theorized that the Roman mixed constitution in which powers were separated was a key factor in the success and stability of Rome. His thought influenced the American founders both directly and indirectly.
It's interesting to observe how the institutional and technological changes since the American founding, and the systemic problems we easily note, align with his discussion. He theorized that each of the unmixed systems had weaknesses that led them to eventually be supplanted by another form, initially well-administered, which too would in time succumb to its inherent weaknesses. The mixed form resisted this cycle. So to the extent that the polity "un-mixes" the arrangement of powers, e.g. making the Senate directly elected instead of chosen by state legislatures without making a corresponding anti-democratic change, the system becomes more exposed to the cyclic risks associated with the unmixed state that was favored.