Yet it was the South that seceded, citing the preservation of slavery, not tariffs as the reason for secession.
You are taking 200 million dollars away from the control of wealthy and powerful interests in the North, and would you rather have them paying attention to that, or would you prefer they think you are doing it for some philosophical disagreement?
Paul Craig Roberts argues the assertion that "slavery" was the issue was just an effort to make a legal case for getting out of the contract.
It may have been that, but I think it was more likely intended as a naive effort at misdirection.
"Oh! We can't live together because we are so different on the issue of slavery! Therefore we will separate from you, and you can feel morally fulfilled because you no longer have to concern yourselves with slavery in your country!" (And by the way, we are taking 200 million per year out of your control. Maybe more even, but pay attention to the slavery thing, not the money thing.)
If you think the North cared more about the issue of Slavery than they did that 200 million, you are naive. If the South thought they could misdirect the North over a moral issue and thereby get them to ignore the loss of all that money, they were very naive.
Yet that was a pretext. Violations of the fugitive slave clause of the Constitution were unconstitutional. Imposing crushingly high tariffs - though bitterly hated - was not unconstitutional.