Answer, the 1820 Missouri compromise.
Reasoning: in 1776 all colonies were slave holding. This practise was carried on into the Articles of Confederation Government of the United States, and even into the 1787 Constitution of the United States which abolished the previous Government. However, under the Constitution state rights were given to the States to made the determination of abolishing slavery within each state.
Several of the northern states did abolish slavery as individual and independent states. However, the question of a balance of powers debate lead to the adoption of the Missouri Compromise which required the new states enter the Union in pairs, one slave state paired with a free state. This effectively eliminated the possibility of one side or the other obtaining a 2/3 majority of states in Congress in order to pass an amendment to the Constitution to eliminate slavery, "Constitutionally".
The elimination of both Missouri Compromise and simular legislature would have allowed one side or the other to have naturally gained the majority to pass such an amendment. The Dred Scott SCOTUS decision eliminated this roadblock but was not given the time to mature and achieve such a majority one way or the other with the admission of new states in the west.
Lastly the War was not fought over slavery, but over secession. Republican Party Platform 1860. Keeping the country together "Preserving the Union" was the main party plank which gave Lincoln the election in the Northern states. It was Lincoln opposition to the secession of South Carolina which triggered the War.