Final answer:
The Embargo Act's main unintended consequence was severe economic damage to the American economy, causing increased unemployment and bankruptcies, and political backlash leading to its repeal in 1809.
Step-by-step explanation:
The main unintended negative consequence of the Embargo Act instated by Thomas Jefferson was the substantial harm it inflicted on the American economy. Instead of coercing Britain and France into respecting American neutrality by ceasing the harassment and impressment of American sailors and shipping, the Act severely damaged American commerce. American merchants were unable to sell their goods overseas, leading to a significant rise in unemployment and bankruptcies, particularly in seaport cities. The drastic decline in business activity and economic hardship at home led to political repercussions and the eventual repeal of the Embargo Act in 1809 after it was deemed a failure.