Please consider this:
In 1856, "Dough-Faced" Northern Democrat James Buchanan was elected president by winning Pennsylvania, Indiana, Illinois and California, plus every Southern state.
He immediately went to work to support the Supreme Court's Dred Scott decision.
Just one more election like 1856, and one more Dred Scott type Supreme Court decision would have effectively made slavery legal in all states.
That's how close the slave-power came to victory.
But in 1860, many people who had voted for Democrats all their lives switched to other parties, because the majority Democrats had committed political suicide.
I'm saying a united, enthusiastic Democrat party in 1860 had a fighting chance to keep those "swing voters" in the Democrat fold, and win the election by carrying those same states it won in 1856.
Not comparable whatsoever. 1856 - you had the Whig and Republican parties divided.