Many people see Thomas Jefferson's greatest failure to be giving up his fight against slavery. While he saw slavery as "a hideous blot" on humanity and made several attempts at its abolishment, he eventually gave it up, deciding to fight only those battles he could win.
Indeed, as the principal author of the Declaration of Independence, Jefferson included a clause denouncing slavery, but it was thrown out before the document could be approved. Though the fact is still often disputed, Jefferson's presidency was damaged further by the assertion that he was the father of five children by slave Sally Hemings. Finally, Jefferson's institution of the Embargo Act, intended to interrupt the hostilities between Britain and France, crippled the Southern economy, alienating the supporters he had left.