If you want to get to the earliest influences on Jefferson and the other founding fathers, you need to read Cicero. They tended to refer to him as Tully, or Tullius -- his full name was Marcus Tullius Cicero. He wrote brilliantly on political theory, drawing on various Greek political philosophers, and also on his own rich experience. Here's a small taste of how shockingly modern he sounds:
"The national budget must be balanced. The public debt must be reduced; the arrogance of the authorities must be moderated and ontrolled. Payments to foreign governments must be reduced. If the nation doesn't want to go bankrupt, people must again learn to work, instead of living on public assistance."
-- Marcus Tullius Cicero, 55 BC
Here is a partial summary of his life and works. I have had the great good fortune to read a fraction of his total output in the original Latin, and he is simply an astonishingly good writer. He's also an egotistical blowhard, but his writing is so moving and beautiful, and he is so innocent compared to many of his bloody-handed contemporaries, that I forgive the self-aggrandizement totally. By innocent I mean that he didn't resort to bloodshed as his first line of defense, he was not fond of military action and avoided it when he could, he didn't travel with a gang of thugs as his personal bodyguard. He loved the law and lived within it.

I went off a bit overmuch there - just wanted to add him to the influences on our own form of government.