The Thirteenth Amendment to the United States Constitution prohibits slavery.
Section 1. Neither slavery nor involuntary servitude, except as a punishment for crime where of the party shall have been duly convicted, shall exist within the United States, or any place subject to their jurisdiction. Section 2. Congress shall have the power to enforce this article by appropriate legislation.
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The delegates at the Annapolis Convention declared slavery to be unconstitutional.
The Liberty party wanted to end slavery by having it declared unconstitutional. The Liberty party argued that enslaving a person was in direct conflict with the United States constitution.
It was only unconstitutional if you accepted Roger Taney's interpretation of the Constitution in his judgment of the Dred Scott case in 1857. He said the Constitution protected slavery - so therefore no state could declare itself to be free soil.
The Judicial Branch
Slavery has not been abolished though some countries have made it illegal.