Why did the federalist party lose support after the war of 1812?
it had opposed the war.
it had been right about the impact on trade.
it had sided with great britain.
it had worked with american indians.

The Federalist Party lost support after the War of 1812 because it had opposed the war.

Explanation:

The Federalist Party, since its inception, showed some sympathy for Britain. Their leaders considered that, although the nation was born after a war with that country, there were more issues that united America with Britain than those that separated both countries. According to the Federalists, America had similarities with Britain, both ideologically and culturally.

Since the confrontation between France and Great Britain during the Napoleonic Wars, this Federalist support increased, compared to the Democratic-Republican support for the French.

When the Democratic Republicans came to power and faced Britain, first diplomatically and then through direct actions, the Federalists opposed the war, protested before it, and even threatened to remove support from the government. This earned them a bad reputation, which meant that, in the face of the development of the actions and the American victory, they lost a lot of voters and support, leading the Federalist Party to its disappearance.