OK, let's do this "by the numbers":
In 1856, a united Democrat party won the presidency with 1,835,000 popular and 174 electoral votes, 22 more than needed.
In 1860, a divided Democrat party lost the presidency -- even with 2,228,000 vs. Republicans' 1,866,000 popular votes -- but Dems got only 84 electoral votes, or 68 less than needed.
In other words, even though Democrats got more votes than Republicans, they still lost in 1860, because they were split in half.
But those 68 electoral votes were easily available in seven states (California, Illinois, Indiana, Kentucky, Oregon, Tennessee and Virginia) with a combined switch of just 11,000 popular votes!
That's because: California (4), Kentucky (12), Oregon (3), Tennessee (12) and Virginia (15) needed zero extra votes for Democrats to win, only a united ticket.
Right there is 46 of the 68 needed to win, leaving just 22 to pick up.
And 24 electoral votes were available in Indiana (13) and Illinois (11) with a switch of only 11,000 votes combined, or 3.5% of the 311,000 cast for Republicans.
Surely a united and enthusiastic Democrat party could persuade 3.5% of "low information" Republicans to switch to the "party of national unity and peace."
But additionally: in Ohio with 23 electoral votes, only 11,000 voters (5%) switching would give Democrats the win.
And in Pennsylvania with 27 electoral votes, only 36,000 voters switching would give Democrats the win.
Yes, that's 13% of all Republicans, but remember in 1856 the now defunct Whigs got 82,000 votes, and at least half of those should have gone to Democrats in 1860.
But instead, the divided Democrats lost 35,000 votes compared to 1856.
That's why I say: victory was not only possible for united Democrats in 1860, they had the potential for a landslide.
But Southern Fire Eaters did not want victory or landslide, rather they wanted secession, and so they engineered it by splitting the majority Democrat party in half.
Again, redo the math. There is simply no way for the democrats to win (or for that matter, any of them), without PA, OH, and NYC. 85 EC votes right there, and don’t forget, that SC was excluded at -8, etc.
The South understood the repercussions of this split and understood that there simply were not enough southern votes to carry the presidency post 1860.
That is one of the reasons they split. To govern themselves, which would not happen in the US after 1860 had they remained.