Why did Jefferson decide that the Louisiana purchase was constitutional?

Thomas Jefferson believed that the Louisiana Purchase was constitutional because he felt it was within his power as President to make such a purchase. He based this opinion on a combination of the Necessary and Proper Clause of the Constitution the precedent of past purchases and a belief that the Constitution was a document of enumerated powers that gave the President certain implied powers.

• Necessary and Proper Clause of the Constitution - Under this clause Congress is allowed to pass laws that are necessary and proper for carrying out its enumerated powers. Jefferson felt that this clause could be interpreted to include the power to purchase land from foreign nations.
• Precedent of Past Purchases - Jefferson noted that previous Presidents had completed land transactions with foreign powers and believed that this established a precedent for conducting such transactions.
• Enumerated Powers - He also recognized that the Constitution was a document of enumerated powers and that implied powers could be used to complete such transactions.

Ultimately Jefferson believed that the Louisiana Purchase was constitutional because it was within his power as President to make such a purchase. He felt that such a transaction was necessary and proper for carrying out the enumerated powers of the Constitution and that the precedent of past purchases established a basis for conducting such a transaction.