1860 is objectively the correct answer. No other electoral contest has so fully and irrevocably decided the national character.
No that's 1800. 1860 had the most important nominating convention as who the Republicans chose would have a major impact, but the South acting like spoiled brats was inevitable that year.
Is not that nominating convention a part of the election? It's beyond dispute that the South was going to try and break off from the Union in 1861 no matter what.
That assumes that the Republicans win the White House in 1860. Assume for the moment that Democrats either hadn't had their 2/3 rule or Douglas manages to get nominated in Charleston despite it. The result is a Douglas victory:
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That's the issue, though: Douglas could not keep the support of Southern Democrats without giving up the Freeport Doctrine wholesale, and he could not do that without loosing the support of his Northern base. Bear in mind that, to win or even deadlock the electoral college, Douglas would have needed to
improve on his actual performance in Illinois and Indiana; I don't see how he could do that while simultaneously winning over the Breckinridge camp. The two-thirds rule was not what split the Democratic Party; it was the insistence of Southern Democrats on nothing less than total commitment to the unfettered expansion of slavery into the territories. That ticket simply could not carry Illinois or Indiana in 1860 (or even 1856), and popular sovereignty was no longer acceptable to the Davises and Breckinridges of the party. Douglas made his choice in 1858 when he sired the Freeport Doctrine as the antidote to Dred Scott, and as a result was nearly as unpalatable to the Southern states as Lincoln was.
The only quasi-realistic chance of preventing civil war in 1860 was to throw the election to the House and somehow elect Bell as a compromise candidate; but the math and the passions of the times combine to make that scenario, at best, a long shot.
EDIT: Come to think of it, this would make an interesting alt-history timeline on the What If board.