The Louisana Purchase in the eyes of some Federalists was the gateway the"common and frontier people" would need to usurp their political idealogy. Political parties were never part of the founding father's plans, however they were quickly becoming a fact of life in the United States.Some Federalists feared that the people settling in the new territory might not be "suitable" to understand the government. If and when the territory applied for statehood the balance of power would definitely be disrupted. The irony of the Louisana Purchase was that the "Anti-Federalist" Jefferson was acting extremely "Federalist" when he stretched the power of the presidency in one of the greatest real estate deals in U.S. history.
There were a number of reasons Federalists opposed the Louisiana Purchase. One can assume some opposed it just because it was Jefferson who made the purchase, out of a kind of reflex. Others had heard wild stories about the land purchased, and thought the purchase a bad bargain. Some thought that the inclusion of Louisiana as the treaty was written would undercut the power of the states by not having each state vote, thus undercutting Federalist power. (Some Federalists in the eastern states specifically feared a shift in power to the west.)
The Federalists opposed the Louisiana Purchase because they better favored close relations with Britain, and not with France, under Napoleon, believing the treaty to be unconstitutional, and more likely spending tons of money to use it as a opportunity to attack Spain if it successfully purchased Louisiana. They feared the rising power from the West, threatened by the new citizens there, so rejected the proposal, fearing political power from the Atlantic seaboard states within the perimeter. Of course, some of them hate Jefferson, who is a non-Federalist so is just pure dislike of the other political group.