Rubbish.
In 1860 there were huge & growing numbers of abolitionists in Southern Border States and Upper South.
Slavery itself in many of those states was declining in numbers or percentages.
That's the reason those states were all reluctant, or flat-out refused, to declare secession in 1861.
But slavery in the Deep South was a very different picture.
In the 1860 Deep South, where cotton was king, slavery had never been more profitable, their economy never more prosperous and slave prices never higher.
So, in the Deep South, to be openly anti-slavery was considered economic & political treason, punishable.
That had nothing to do with Northerners, but rather with the obviously manifest self-interests of huge Deep South slave-holding classes.
Consider the contrast: in Border States, about 15% of white families owned slaves, and majorities were pro-Union, whereas in the Deep South, nearly 50% of white families owned slaves and majorities were easily susceptible to secessionists' propaganda.
So, among our Founders in 1787, Southern leadership (i.e., Washington, Jefferson, Madison) recognized slavery as morally wicked, but economically necessary.
And by 1860 many Southerners still felt that way, though not in the Deep South.
There, amazing prosperity manifest their beliefs that slavery was not only economically necessary, but divinely instituted.
That's why they would, by God, tolerate no criticism of slavery.