[LS]

Yes, I know MVB was a New Yorker, hence a "northern man of southern principles." In fact, technically he was still a slaveholder, although his one slave---given to him as a teenager by his dad---had run off. But MVB never officially freed him nor searched for him.

As to who would have won without a split into four parties, there are two ways of looking at it: if you combine northern Dems, Republicans, and the Constitutional Union Party as effectively voting against slavery, then it's more than 2/3 of the voters rejected slavery. If you view the Dems (even northerners) as a pro-slavery party, then it's closer to 50-50.

[BroJoeK]

LS: "if you combine northern Dems, Republicans, and the Constitutional Union Party as effectively voting against slavery, then it's more than 2/3 of the voters rejected slavery."

Hmmmmmmm....
I'm pretty sure you know that in 1860, virtually nobody "rejected slavery" or "voted against slavery."
Slavery itself was not on the ballot in 1860.
What was, in effect, "on the ballot" was the expansion of slavery, especially as represented by the 1857 Supreme Court's Dred-Scott decision, making it more difficult for non-slave states to effectively outlaw slavery within their own borders.
Politics in 1860 can be seen as the country's response to the Supreme Court's over-reaching support for slavery in 1857.

Indeed, you could well say that the 1860 election was all about States' Rights -- the rights of non-slave states to effectively outlaw slavery within their borders, as opposed to the Federal Government's power to enforce pro-slavery laws in those non-slave states.

And the key to understanding what happened in 1860 is to study the electoral results of 1856, because that was the base from which the dominant majority Democrat party began, and which the Southern Fire Eaters utterly destroyed by walking out of their April 1860 Democrat convention.

1856 results, blue = Democrats, this shows the epitome of the alliance between the Southern Slave Power and Doughface Northerners:

Note that in 1856, the total Democrat popular vote was only 45%, and yet they easily won the electoral college.
In 1860, Democrat popular vote grew to 48% -- near an absolute majority, and yet Democrats lost in 1860 to the minority Republicans -- largely because Southern Fire Eaters split the Democrats in half.
I'm simply arguing that 1860 could have been a repeat of 1856, had the Dems just played their cards right.
So I'm saying, in 1860 the voters themselves didn't change.
What changed was the choices they were offered -- choices in effect engineered by Southern Fire Eaters.