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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.