Complex sentence: His 1784 Ordinance would have prohibited slavery completely by 1800 in all territories, but was rejected by the Congress by one vote due to an absent representative from New Jersey.
Split into simpler sentences: His 1784 Ordinance would have prohibited slavery completely by 1800 in all territories. Congress, however, rejected Jefferson's original 1784 Ordinance prohibiting slavery by only one vote, due to an absent representative from New Jersey.