THE US CONSTITUTION ARTICLE 4 SECT 3 PARAGRAPH 3: No Person held to Service or Labor in one State, under the Laws therof, escaping into another, shall, in Consequence of any Law or Regulation therein, be discharged form such Service or Labour, but shall be delivered up on Claim of the Party to whom such Service or Labour may be due.
And for ending Jim Crow it was the belated enforcement of the 14th Amendment 1st section. See the underlined.
All persons born or naturalized in the United States, and subject to the jurisdiction thereof, are citizens of the United States and of the State wherein they reside. No State shall make or enforce any law which shall abridge the privileges or immunities of citizens of the United States; nor shall any State deprive any person of life, liberty, or property, without due processof law; nor deny to any person within its jurisdiction the equal protection of the laws.