Thoughts on the US Constitution:
The US Constitution owes far more to Rome than to Athens. The laws of Solon established merely a popular government which declined into tyranny(Peisistratos) in less than a generation. By contrast, Rome's constitution was the true seed of liberty and the longest surviving political institution in the history of mankind.
The Roman constitution wasn't a written constitution, however upon the creation of the Tribunes a "mixed government" system came into being. The Consuls had inherited the monarchical power from the kings, the Senate retained the aristocratic power and the through the magistracy of the Tribunate the people were enfranchised. As John Adams said:
"Without three divisions of power, stationed to watch each other, and compare each other's conduct with the laws, it will be impossible that the laws should at all times preserve their authority, and govern all men."
Rome also developed what we now call a "separation of powers" - executive, legislative and judicial branches of government and what we now call "checks and balances" to keep these branches from gaining dominance over the others.
The Roman constitution was not static and unchanging but developed over time. In Discourses on Livy, Machiavelli describes how the corruption of the Roman people led to constitutional reforms:
'It was necessary therefore, if Rome wished to preserve her liberty in the midst of this corruption, that she should have modified her constitution, in like manner as in the progress of her existence she had made new laws; for institutions and forms should be adapted to the subject...'
The reforms of Rome's constitution were easier as Rome's constitution was not a written constitution. As Machiavelli explained:
'...the majority of men never willingly adopt any new law tending to change the consitution of the state, unless the necessity of the change is clearly demonstrated; and as such necessity cannot make itself felt without being accompanied with danger, the republic may easily be destroyed before having perfected its constitution'
The 'danger' that presented itself to the founding fathers was a weak executive branch and the tyranny of the states and the people. Adams was acutely aware of this; Jefferson was not.
Adams advised the drafting committee: "it is of great importance to begin well; misarrangements now made, will have great, extensive, and distant consequences; and we are now employed, how little so ever we may think of it, in making establishments which will affect the happiness of an hundred millions of inhabitants at a time, in a period not very distant."
After signing the constitution Adams wrote to Jefferson: "you are afraid of the one, I am afraid of the few...I would therefore have given more power to the President and less to the Senate." Adams also didn't believe Presidential appointments should need to be approved by the Senate.
Adams was adamant: "'If there is one central truth to be collected from the history of all ages, it is this: that the people's rights and liberties, and the democratical mixture in a constitution, can never be preserved without a STRONG EXECUTIVE."
 Rule of the one - monarchy
 Rule of the few - oligarchy
 Rule of the many - democracy/anarchy