Can you imagine how galling it must have been for northerners to have hostile and belligerent bands of mercenaries tramping through their communities, seizing what they generally regarded as citizens off the street, and effectively kidnapping them? That was the “fugitive slave act” in action.
By 1800 every northern state had either outlawed slavery or had defined a path to eventual emancipation. They increasingly regarded the practice as barbaric and wanted nothing to do with it. Lincoln came on the scene proclaiming that it was not his intention to interfere with the south’s Particular Institution except to prevent its westward expansion. In that he was little different than any of the previous northern presidents.
The southern democrats, being democrats, overreacted and overreached as is typical of their nature. And they paid a terrible price for their poor choices.
Many of the prewar presidents were all for the expansion of slavery, notably Polk, who arguably started the Mexican War primarily to get territory to expand slavery into. Which didn’t work out too well. Buchanan of course did everything he could to sneak Kansas in as a slave state. (With a constitution specifying capital punishment for expressing abolition sentiments.)
They didn’t like it, but most of the people and politicians of the North were willing to put up with it in order to avoid war. Were there any concessionary gestures made by the South in order to avoid war?