During the pre-Civil War slavery era, there were a number of abolitionists such as
Samuel May (1836),
George Mellen (1841),
Alvan Stewart (1845),
Joel Tiffany (1849),
William Goodell (1852),
Benjamin Shaw (1846),
Edward Rogers (1855), and
Lysander Spooner (1845), who wrote on slavery's unconstitutionality.
They said that pursuant to common law, centuries of precedents, and constitutional and legal principles dating back to the Magna Carta (banning detentions without due process, e.g., banning detentions without charges verified by conviction in a jury trial), slavery was unconstitutional, and illegal as well, pursuant to anti-kidnaping laws.
Preparatory to your reading this site, reading the historical and constitutional law overview, and/or some or all the above authors' writings, may be helpful.
Nathaniel P. Rogers (1794 - 1846) was an abolitionist attorney and publisher.
This site reprints his 1837 "The Constitution" article, on slavery unconstitutionality, from the Quarterly Anti-Slavery Magazine, pp 145-153.
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Biography (by Pillsbury)