White Americans of both sections believed, at least from the Mexican War onwards, that things were growing increasingly dire and the nation may not be able to contain the division. Sectional strains were older than the Union and they had always been contained before, generally with great cost to freedom and trifling concessions by the slave states, but it was managed.
The multiyear struggle over the fate of slavery in the Mexican Cession marks a significant shift. By the time things were settled to no one's satisfaction in late 1850, the coalition was so openly fragile (something like five senators voted for the entire Compromise of 1850) and both sections largely opposed to the settlement that things never quite returned to the older status quo where slavery was a usually unacknowledged central issue of the nation's politics. Even proposals to endorse the Compromise measures as the final sectional covenant, forever settling the nation's slavery problem, drew controversy both over their popular repudiation in both sections and fears that just talking about them would reopen the issue and make things worse.
There are still peaks and valleys, of course. The Compromise measures prompt an immediate backlash, which includes an abortive secessionist conspiracy in the South (Historians remain divided on how realistic it was.) and widespread opposition to the Fugitive Slave Act of 1850 in the North. This was a critical issue because the secessionists in the three states conspiring then (South Carolina, Georgia, and Mississippi) foundered in part due to Unionists putting forward the Georgia Platform. Originally from that state, it basically declared the South's acceptance of the Compromise of 1850 with conditions. First, they did not like it one bit. Second, their continuance in the Union depended on the good behavior of the North, most especially the zealous enforcement of the Fugitive Slave Act.
The North's good behavior with regard to that law rose to the level of violent resistance. There are cases of antislavery whites and blacks joining together to free accused runaway slaves from courthouses, as well as shootings in more remote locales. In Upstate New York, a slave catcher tried to take a man from a town near to where an antislavery convention was then taking place. They adjourned to bust him loose. We can't know how many enslavers tried to recapture people (or just kidnap free people outright) but were deterred by the resistance. The actual clashes are easier to document and many, but I have a whole lot of ground to cover here so I'll just go into detail with one.
The Fugitive Slave Act of 1850 passes in September of 1850. Shadrach Minkins, enslaved in Virginia, stole himself to freedom in May of that year. He ended up in Boston (probably arriving by sea) and became a waiter at Taft's Cornhill Coffee House, a nice joint less than a block from the courthouse.
Shadrach legally remained the property of John DeBree, US Navy. DeBree paid good money for Minkins only the year before and wanted him back. He hired a man named John Caphart to act as his agent. He got into Boston on February 12 and spent three days getting the legal ducks in a row. Then two men went off to the coffee house. Minkins waited on them and they seized him. From there it was straight to the federal courthouse, because Massachusetts law didn't allow the use of state buildings for slave renditions. Minkins did not immediately put up a fight, but once he realized what was up he made a scene and got noticed.
Boston was the most infamously antislavery city in America at the time. Black and white abolitionists and antislavery people quickly involved themselves. According to the law, you had to get a hearing before a special commissioner before you could get the legal go-ahead to remove someone back to slavery. The commissioner had to agree that you had the right person and had a legal right to them, which was easy to establish because the accused had no right to speak in their own defense and, incidentally, comissioners got paid $10 a head for every person sent South but only $5 if they ruled in favor of freedom.
The hearing convened with more than a hundred people in the audience. Minkins had six lawyers to work his defense. The court agreed to give them three days to ready one, which he didn't technically have much right to under the law. (The Fugitive Slave Act was a stunning piece of work that way.) During that time, he would remain in custody at the courthouse. All that settled, they hung around in the courtroom to begin planning things. It emptied out a bit and some people came and went.
Outside, an abolitionist mob formed. Hundreds of people turned out demanding his release. The lawyers had three days to secure Minkins' freedom. The mob took three hours. Then about twenty black men charged the courtroom, seized Minkins, and got him out. They hid him in an attic, then got him to Cambridge, thence to Concord where I've heard there's some dispute over just which building housed him, and finally he made it to Canada. He settled in Montreal.
Shadrach Minkins was the last fugitive slave anyone attempted to recover from Boston until 1854. Then a broadly similar series of events played out, including the same courthouse. This time the Massachusetts militia turned out to defend it and a deputy federal marshal was killed in the line of duty doing the same but that enslaved man, Anthony Burns, was returned to slavery courtesy of a large contingent of the United States Navy and Marine Corps, by way of a revenue cutter rerouted specifically to do the job.
By the time Burns was taken back to slavery (also in Virginia, incidentally) the infrequent and sporadic violence attending fugitive renditions was set to get a much higher-profile competitor. Franklin Pierce had just signed the Kansas-Nebraska Act into law, repealing the Missouri Compromise and thus opening the remainder of the nation's territories (roughly everything between Minnesota, Iowa, and Missouri and the Rockies) to slavery. The two events happen literally within days.
This was not something that white Northerners took very well. To them, that was their land. They had it promised to them thirty years before. Yet now it could have slavery or not, depending on how a vote of the settlers went. So they decided they would be those settlers and organized subsidized settlement there. Fair's fair, right?
Not so. Kansas directly abutted Missouri's most enslaved region. That area was full of people who believed it belonged to them. The way they read the law, slavery was instituted in Kansas until voted out and they were going to make sure that never happened...even if they had to send a few thousand guys with guns, knives, and a couple cannons across the border to vote illegally in the territory's elections. So they did.
The violence goes back, on a small scale, to Kansas' very first election. This was a non-event, politically, electing a single delegate to Congress who couldn't vote and would get to Congress just in time for his term to expire. Yet some Missourians came over and roughed up people who objected to their voting illegally. Things get much worse with the legislative elections in 1855, where almost every seat in the legislature was decided by illegal voting. In a few cases, the governor set those outcomes aside and ordered do-overs. He announced those rulings with an armed bodyguard, to a crowd of armed proslavery men.
The legislature convened and the majority, still easily proslavery, voted to expel the few antislavery members elected at the special elections in favor of the originals elected by fraudulent votes...which had been backed by the threat and occasional commission of violence. The proslavery legislature promptly comes to loggerheads with the governor, which it gets fired, and passes a draconian set of laws that basically outlaw being antislavery in Kansas.
By the late fall, antislavery Kansans and slavery-indifferent or proslavery Kansans who still think this is all completely a unacceptable way to treat self-governing white men get together and start up an illegal state government of their own. They organize a political party and write a constitution, which they get ratified by the population (illegally) in December. While this is going on, a proslavery Kansan murders an antislavery Kansan (Who had probably burned a few proslavery settlers out of their houses, I'm eliding a lot of small-scale violence and a few murders here). The local antislavery militia group gets wind of this because one of its officers literally lives next door and was friends with the murdered guy, and soon there's a manhunt on.
The new governor of Kansas, much more proslavery than the last, responds with a manhunt for said officer, who is taken by a sheriff but then rescued by the antislavery militia. (This is just as convoluted as it sounds.) It all leads up to a proslavery army of around two thousand camped around Lawrence, headquarters of the Kansas antislavery movement, and aiming to burn it all to the ground. Opposing them are 800 or so antislavery militiamen armed with a cannon or two of their own. They very nearly make a fight of it, but protracted negotiations eventually lead to everyone going home.