The Fugitive Slave Act proved the most controversial element of the compromise. The act required federal magistrates to determine the status of alleged runaways and denied them a jury trial or even the right to testify. Using its provisions, southern owners re-enslaved about 200 fugitives (as well as some free blacks).

The plight of the runaways and the presence of slave catchers aroused popular hostility in the North and Midwest. Ignoring the threat of substantial fines and prison sentences, free blacks and white abolitionists protected fugitives. In October 1850, Boston abolitionists helped two slaves escape from Georgia slave catchers. Rioters in Syracuse, New York, broke into a courthouse, freed a fugitive, and accused the U.S. marshal of kidnapping. Abandoning nonviolence, Frederick Douglass declared, “The only way to make a Fugitive Slave Law a dead letter is to make half a dozen or more dead kidnappers.” Precisely such a deadly result occurred in Christiana, Pennsylvania, in September 1851, when twenty African Americans exchanged gunfire with Maryland slave catchers, killing two of them. Federal authorities indicted thirty-six blacks and four whites for treason and other crimes, but a Pennsylvania jury acquitted one defendant, and the government dropped charges against the rest.

Harriet Beecher Stowe’s novel Uncle Tom’s Cabin (1852) boosted opposition to the Fugitive Slave Act. Conveying the moral principles of abolitionism in heartrending personal situations — using the now familiar literary trope of sentimental domesticity — Stowe’s book quickly sold 310,000 copies in the United States and double that number in Britain, where it prompted an antislavery petition signed by 560,000 English women. As Uncle Tom’s Cabin sparked an unprecedented discussion of race and slavery, state legislators in the North protested that the Fugitive Slave Act violated state sovereignty, and they passed personal-liberty laws that guaranteed to all residents, including alleged fugitives, the right to a jury trial. In 1857, the Wisconsin Supreme Court went further, ruling in Ableman v. Booth that the Fugitive Slave Act was unconstitutional because it violated the rights of Wisconsin’s citizens. Taking a states’ rights stance — traditionally a southern position — the Wisconsin court denied the authority of the federal judiciary to review its decision. In 1859, Chief Justice Roger B. Taney led a unanimous Supreme Court in affirming the supremacy of federal courts — a position that has withstood the test of time — and upholding the constitutionality of the Fugitive Slave Act. By then, as Frederick Douglass had hoped, popular opposition had made the law a “dead letter.”