The Morrill Tariff passed the U.S. House of Representatives on May 10, 1860, before Lincoln's election and before any state had seceded. It passed the U.S. Senate on March 2, 1861, two days before Lincoln's inauguration. (Lincoln vigorously lobbied for the bill, telling a Pittsburgh, Pa. audience two weeks before his inauguration that no other issue — none — was more important.)
On November 19, 1860 Senator Robert Toombs gave a speech to the Georgia convention in which he denounced the "infamous Morrill bill." The tariff legislation, he argued, was the product of a coalition between abolitionists and protectionists in which "the free-trade abolitionists became protectionists; the non-abolition protectionists became abolitionists." Toombs described this coalition as "the robber and the incendiary... united in joint raid against the South."
The Morrill tariff's passage in the Senate was a certainty. All that was needed was to flip one or two senators. This could easily be done by offering a key concession or two and threatening that if they did not agree to it and some other senator did, then his constituents would get whatever was offered to them in exchange for his vote and the offers made to other senators and their states would be withdrawn. The Morrill Tariff was the main political issue throughout the Fall of 1860.
Several English publications at the time. Chamber’s Journal of Popular Literature, Science and Arts, one of the “workingman’s journals,” wrote on March 21, 1857, that a major source of conflict was that Northern business interests wanted the South to “consent to the high protective tariff,” and if they did, “anti-slavery agitation would stop.” “Antislavery agitation” meant opposition to the extension of slavery, not Southern slavery. Pretending to want to “check the progress of slavery” in this way “has been only a disguise under which to advance the interests of the [Republican] party.”
The Edinburgh Review was a prominent British journal that observed in 1858 that “abolition was not a policy of the North,” and that secession would actually spell the end of slavery because it would no longer be propped up by the federal government’s Fugitive Slave Act. This view was echoed by other high-quality British publications such as Fraser’s Magazine and The Saturday Review, among others. Thus, the most prominent British journals agreed on the eve of the War with a statement that Alexander Stephens would make five or six years later, that slavery was actually “more secure” in the union than out of it. see the statement of Lincoln and others which I cited earlier. This was not a fringe view nor one that only Lincoln could see.
The Quarterly Review agreed wholeheartedly with Dickens, calling the protectionist tariff a “revolting tribute” paid to Northern businessmen by Southerners who “had been groaning for years under the crashing bondage of Northern protectionists.”
Blackwood’s Magazine, which is still being published, argued in 1861 that “slavery had no significant part in the conflict.” The union, through the Fugitive Slave Act, protected slavery, said Blackwood’s, repeating the view of other British journals that secession would actually lead to the demise of slavery by nullifying that federal law. The tariff laws, on the other hand, were “ruinous to the South.” They were “the chief complaint of the South,” and “have been for thirty years oppressive and unjust.”
Similarly Robert Barnwell Rhett aka "the Father of Secession" a few years before the war stated: "The legislation of this Union has impoverished them [the Southern States] by taxation and by a diversion of the proceeds of our labor and trade to enriching Northern Cities and States. These results are not only sufficient reasons why we would prosper better out of the union but are of themselves sufficient causes of our secession. Upon the mere score of commercial prosperity, we should insist upon disunion. Let Charleston be relieved from her present constrained vassalage in trade to the North, and be made a free port and my life on it, she will at once expand into a great and controlling city."
US Senator from South Carolina James H. Hammond likewise stated in 1858, "I have no hesitation in saying that the Plantation States should discard any government that makes a protective tariff its policy."
No, the Morrill Tariff did not magically become an issue on the day it passed the Senate and was signed into law by the president. It had been the major issue in the country for the previous year.
Picking and slinging about bit and pieces of opinion at the margins does not change the fact that slavery was the cause of secession, not tariff rates.