Answer:
He feared tyranny of any kind and only recognized the need for a strong, central government in terms of foreign affairs. He was concerned that the Constitution did not address the liberties that were protected by the Bill of Rights and did not call for term limits for the president.
Politically, Jefferson did not favor a strong central government. Conversely, he envisioned a country of yeoman farmers, to whose judgment most matters could be referred, i.e. strong local (state) governments. To his way of thinking (and what became the core of the Democratic Party for the next half-century), the Constitution assigned specific functions to the federal government. States Rights prevailed otherwise. Thus Congress, elected by the states, was dominant.
But perhaps the most “simplistic” factor was the deadline for the purchase: the offer expired on October 31. Use it or lose it. Deeply torn between his personal (and well known) ideology and a phenomenal deal, he had to jump on it!
Kit and caboodle, it was negotiated down to only $15 million.