Why was the Louisiana Purchase controversial?

The controversy didn't end there. People were already living in the Louisiana territory, most of them French, Spanish, or free Africans ... Since the Louisiana Purchase was part of a treaty with Napoleon that Jefferson was entering into, it could not be unconstitutional."

Many of President Jefferson’s contemporaries felt that he overstepped his Constitutional authority as President of the United States by buying the land from the French. Some also felt that buying from the French would alienate the English which would hamper their attempts to make England a close ally. Some smaller concerns also included the Eastern states could lose their power as people moved west, a decision on if the people already living in Louisiana would become citizens, and of course the issue of slavery in the region. What got the Purchase passed through Congress was that it was part of a treaty with Napoleon and Constitutionally the President can make treaties.