Why did President Jefferson have doubts about buying the Louisiana Territory?

Jefferson's only real doubts were that the Constitution might not actually allow the president to make such a purchase and that there would be political fallout for his Republican Party.

Jefferson had been, for the most part, a strict constructionist when it came to the constitutional powers of the 3 branches. The purchase would require Jefferson to take a rather broader interpretation of the president's powers. In the end, the purchase was such a bargain (822,000 square miles for $15 million) and so critical to U.S. expansion (doubling the size of the country) that few complained seriously about Jefferson's action.