The sectional insults do not mirror each other precisely. Antislavery politics meant far less to most Northerners than they did to Southerners who could, with some justification and some racial panic, see them as mortal threats. But Wilmot proposed only banning Southern expansion. James Mason’s Fugitive Slave Act required exporting Southern law and white slavery culture to the North. It reached into the present lives of Northerners going about their everyday business in a way that Wilmot never dreamed of intruding upon the South.
The slanted nature of the Fugitive Slave Act’s processes turned swiftly from speculation into reality. James McPherson notes in Battle Cry of Freedom:
In the first fifteen months after its passage, eighty-four fugitives were returned to slavery and only five released. During the full decade of the 1850s, 332 were returned and only eleven declared free.
The spectacle of long-settled people spirited away by slave-catchers and federal marshals did not long remain theoretical either. McPherson lists several:
In September 1850, […] a black porter who had lived in New York City for three years and took him before a commissioner who refused to record the man’s insistence that his mother was a free Negro.
In February 1851 agents arrested a black man in southern Indiana, while his horrified wife and children looked on, and returned him to an owner who claimed him as a slave who had run away nineteen years earlier.
A Maryland man asserted ownership of a Philadelphia woman [Euphemia Williams] who he said had run away twenty-two years previously. For good measure, he also claimed her six children born in Philadelphia. In this case the commissioner found for the woman’s freedom.
The porter had friends who raised the money to buy him freedom and the Philadelphia woman beat the odds, but few had such good fortune. Indeed, many slave-catchers simply didn’t bother with the formality even under a system that wrote them virtually carte blanche but instead seized who they wanted and started South at full speed. Seeing these spectacles and others put the white North in a spot where its own complex loyalties to various principles of justice, due process, rule of law, and so forth came into conflict with slavery.
With the burden on the accused to prove his or her innocence but all means to do so specifically denied, even white Northerners who deeply disliked living near to black people might face the spectacle of someone who, almost unwittingly, had become a person in their minds suddenly spirited away on no more than a slave catcher’s say-so. Had Ona Judge not been claimed by old age, the Custis heirs could have come to New Hampshire to reclaim her from fifty years of freedom.
Faced with the blatant unfairness and all this spectacle, many Northerners chose resistance, or at the very least standing aside when their abolitionist brethren did.