[LS]

Pretty much I guess we agree. But again, you have to see Huston’s book to see, regardless of what the Republicans said, how inevitable the fight over the definition of property rights was and how, eventually, this would force the Republicans to act. Hey, I’m not trying to argue that the North was at fault, but I do not think the South’s interpretation of the near future was wrong. When you say “we lasted 70 years,” well, that included a ban on doing anything about the slave trade for 20 of those years, then of the remaining 50, the Van Buren party system accounted for 42 of them. The surprise is not that it fell apart, but that Van Buren’s shenanigans actually kept it together so long.

[BroJoeK]

LS: "The surprise is not that it fell apart, but that Van Buren’s shenanigans actually kept it together so long."

Something fundamental had changed between the Democrat victory in 1856 and their downfall in 1860.
What was it?

Answer: First of all, the Supreme Court's 1857 7-2 Dred-Scott Decision denying the rights of non-slave states to grant citizenship to people of African descent.
This decision, supported by Doughface President Buchanan was a huge victory for "Southern principles", and it fundamentally changed both sides:

• The South felt vindicated in their belief that the Constitution allowed for no -- zero, zip, nada -- interference with slavery.

• The North felt outraged that their slow but steady efforts to abolish slavery were now virtually invalidated.

So Fire Eaters felt emboldened to expand slavery, while Wide Awakes felt determined to push back.
In 1860 politics played out on a grand scale what citizens felt about Dred-Scott's sudden change to those generations' long compromises which were necessary to preserve the Union.