Why did Federalists oppose the Louisiana Purchase and what did they plan to do about it?

To understand why some Federalists were opposed to the Louisiana Purchase, take a look at the following extract from an editorial written by Alexander Hamilton in 1803:

As to the unbounded region west of the Mississippi... a wilderness through which wander numerous tribes of Indians. And when we consider the present extent of the United States, and that not one-sixteenth part of its territory is yet under occupation, the advantage of the acquisition, as it relates to actual settlement, appears too distant and remote to strike the mind of a sober politician with much force.

In other words, the Federalists opposed this purchase because the land was inhabited by Native Americans and, therefore, the chance of being able to settle people here seemed low, if not impossible. (Read the full source in the first reference link provided).

In response to the president's desire to go ahead with the purchase, some Federalists, like those in new England, threatened to secede from the United States. These threats, however, did not hinder the purchase and, in fact, during a vote in the Senate, the motion passed with an overwhelming majority, at 26 votes to 6. (See the second reference link).