The War of 1812 was unpopular in New England, where it was felt that its commercial interests were being sacrificed by the Southern and new Western states. During the war, Federalists discouraged enlistment and resisted attempts at conscription. The war was devastating to New England shipping and when the British exempted New England from its blockade, the United States imposed an embargo that did their work for them.
On October 18, 1814, the Massachusetts legislature called for a convention to discuss constitutional means to protect New England interests. All New England states were invited to attend. Delegates officially elected by their respective legislatures were sent from Massachusetts, Connecticut, and Rhode Island. Vermont and New Hampshire did not participate as states, although a few counties sent delegates.
On December 15, 1814, Federalist delegates gathered in Hartford, Connecticut, to discuss the impact of the War of 1812 on their home states' economies. There were 26 delegates, almost half from Massachusetts. The meetings were held in secret (similar to the Constitutional Convention). Some discussion of secession was aired, but the major emphasis was to propose constitutional changes to prevent similar crises from occurring in the future.
The delegates drew up proposed amendments prohibiting embargoes of more than 60 days and barring the election of a president from the same state twice in succession (obviously aimed at the so-called "Virginia Dynasty"). The representation of slaves, even at the 3/5 rate embodied in the constitution, was to be ended. The defense of states was to be entrusted to their state governments and that the federal government should allocate some of its revenue for this purpose. The convention agreed to its demands on January 4, 1815, and determined that its emissaries should take them to Washington. It adjourned the following day.
They did not realize that at that time, a ship was sailing across the Atlantic with with news that a peace agreement had been reached in Ghent, Belgium, bringing the war to a close. Before the convention's representatives reached Washington DC to present their demands, news of Jackson's victory at New Orleans raised patriotic sentiment in favor of the war and made them appear foolish and, due to the whiff of secession, perhaps traitorous. Participation in the Hartford Convention was politically fatal for its leaders.