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:
By the way, the Dred Scott decision also triggered the Panic of 1857. Guys in black robes can screw up an awful lot.