Final answer:
The northeastern states opposed the War of 1812 predominantly due to economic harm to their maritime-dependent economies and the perception that the Jefferson and Madison administrations were indifferent to their interests, culminating in the Hartford Convention where secession was discussed.
Step-by-step explanation:
The northeastern states opposed going to war with England during the War of 1812 because it caused significant economic harm to a region heavily dependent on maritime commerce. The war's unpopularity fueled a resurgence of the Federalist Party in New England, where they resented the dominance of slaveholding Virginians like Jefferson and Madison, who seemed indifferent to New England's interests. The Hartford Convention of December 1814 highlighted the extent of this discontent, where Federalist attendees from Connecticut brought up the notion of seceding from the United States. The perception of the Federalists as unpatriotic during the convention, which criticized the government's actions, led to the party's downfall.