I meant for this post to precede the previous but screwed up yesterday. I think it still works as a companion piece, but my apologies for the disruptions in the narrative.
Clay offered the North a free California and no slave trade in the District of Columbia. He bought off Texas, settled its disputed border, and offered up as many as four new slave states for the South.
But the South had one other great concern to compromise away. Southern leaders long feared shrinking. Should it no longer have room to expand, demand for slaves would fall and take their prices with it. The loss of capital would ruin Southern planters. But the South also feared a kind of unwitting expansion by shrinking: slaves stealing themselves to freedom. In Congress during the debates on the Clay Measures, Southerners first inveighed against the Underground Railroad under that name for stealing their property.
The steady bleed of slaves escaping to free states where they often enjoyed limited legal protections, sympathetic white minorities, and supportive free black communities came largely from the Border States. As Border State slave populations shrunk, those states culturally and politically aligned more and more with free states, to the minds of Deep South thinkers. Those men, Calhoun especially, feared that the Border States would follow the example of the Lower North decades prior and free their waning numbers of slaves. The South would shrink.
The Deep South feared not in vain. A single senator gone soft on slavery delivered the Senate to the North with its Wilmot Proviso. A great deal of two-party politics in the South involved arguments over which politician had more loyalty to the peculiar institution. Sometimes less prominent critics of slavery, or just outsiders in the wrong time and place who Southerners could imagine disliked it, ended up lynched to keep order and serve as an example to others. Furthermore one could hardly deny that the Lower North had, in fact, emancipated when its black population declined. More than personal interest in being able to recover human property drove the South. The fugitive slave issue reached deep into the ideological psyches of Southern leaders.
The Constitution required states to hand over fugitive slaves. Law dating back to 1793 put in place mechanisms to enforce that provision. Fugitives under that law for life, any runaway could never be perfectly secure no matter how far he or she ran. Runaway female slaves would transmit their status to any children, just as they would had they never absconded. To claim a runaway, the owner or designated agent simply had to seize the person, haul them before a judge and swear out an oath that the person in hand belonged to him. The accused had no right to as much as speak in defense against being taken. Naturally, this meant that any reasonably dark-skinned person could be stolen South on nothing more than the accuser’s say-so. As slave-catchers had a financial interest in delivery and any “mistakes” could be sold in some distant slave auction, the fugitive slave law legalized kidnapping in all but name.
Those facts sat poorly with the North and Northern legislatures responded with personal liberty laws granting accused runaways jury trials before they could be removed, barring state officials from helping with the arrest or detainment of the accused, forbidding the use of state jails for that purpose, and generally making a slave catcher’s job harder. In 1842 the Supreme Court ruled in Prigg vs. Pennsylvania that while owners had a right to recover their fled property, states did not have to help them do it. Personal liberty laws proliferated, greatly improving the security and safety of fugitives and free blacks in the North alike.
The Great Compromiser took aim at those personal liberty laws with a greatly strengthened Fugitive Slave Law, which would enrage the North nearly as much as the Wilmot Proviso enraged the South.