What event is considered the "nail in the coffin" for the Federalist Party?

The Hartford Convention is viewed as the "nail in the coffin" for the Federalist Party primarily due to its timing and political implications. Held in late 1814 and early 1815 during the War of 1812, the Hartford Convention was convened by New England Federalists who were discontent with the federal government's policies and the ongoing war, which they believed adversely affected their region’s economy.

The convention was controversial and gave rise to the perception that the Federalists were unpatriotic, especially when news of the Treaty of Ghent (which effectively ended the War of 1812) reached the public soon after the convention. The desire of some delegates to advocate for, or discuss, the idea of secession from the Union further damaged the party's reputation. This event led to a significant decline in the Federalist Party's influence. After the war, the country experienced a surge of nationalism, which overshadowed the Federalists' grievances. Consequently, the party struggled to regain support and effectively became irrelevant in national politics, marking the end of its prominence.