That is when it passed Senate approval. It had already passed the House nearly a year earlier in May 1860. A few weeks earlier Lincoln, who had campaigned on a protectionist plank, had pledged to make the new tariff a top legislative priority in the next session if it was not passed by his inauguration. The bill also had the backing of the outgoing president, himself a protectionist from Pennsylvania. In December 1860, the only thing the Morrill bill had not recieved support of was the senate. Rather than being able to block the bill in the senate, documents at the time indicate that the southern senators knew it was inevitable.
Only the walkout of all those Democrats from the Confederate states made its passage possible.
That is simply not so. The following calculations of the south's inability to block northern legislation were listed by Senator Wigfall of Texas on December 12, 1860 before any state had seceded:
"WIGFALL: Tell me not that we have got the legislative department of this Government, for I say we have not. As to this body, where do we stand? Why, sir, there are now eighteen non-slaveholding States. In a few weeks we shall have the nineteenth, for Kansas will be brought in. Then arithmetic which settles our position is simple and easy. Thirty-eight northern Senators you will have upon this floor. We shall have thirty to your thirty-eight. After the 4th of March, the Senator from California, the Senator from Indiana, the Senator from New Jersey, and the Senator from Minnesota will be here. That reduces the northern phalanx to thirty-four.
BINGHAM: Douglas.
WIGFALL? What?
BINGHAM: Douglas.
WIGFALL: Non tali auxilio nec defensoribus istis, tempus eget. There are four of the northern Senators upon whom we can rely, whom we know to be friends, whom we have trusted in our days of trial heretofore, and in whom, as Constitution-loving men, we will trust. Then we stand thirty-four to thirty-four, and your Black Republican Vice President to give the casting vote. Mr. Lincoln can make his own nominations with perfect security that they will be confirmed by this body, even if every slaveholding State should remain in the Union, which, thank God, they will not do." - Congressional Globe, 36-2 p. 74
Wigfall's sentiments on the northern ability to control the senate are evident throughout the session among the southern delegation, including before any state seceded. They were also expressed when the Morrill bill came up for debate in early February at a time when several of the states were in the process of seceding. Here is what Senator Hunter of Virgina had to say on that matter:
"Mr. President, it is very disagreeable to speak, as I do on this occasion, with a consciousness of my utter inability to prevent the passage of this bill. I have no doubt that the adoption of this measure is a foregone conclusion. I believe it has been generally understood that the adhesion of the State of Pennsylvania to the Republican party was upon the condition of the passage of this Morrill-tariff bill; and I suppose an obligation that has been incurred at such a price must be carried out. Still, I owe it, perhaps, to those whose opinions I represented on this committee, and to my constituents, to expose, if I can, the shallow pretexts on which it is sought to adopt this measure, and strip it of those disguises in the shape of specific duties, under which its enormous taxation is hidden."
He spoke at length about the wrongs inflicted by the tariff - 7 pages in the congressional record, not once mentioning slavery - and concluded with the following:
But pass this bill, and you send a blight over that land [of Virginia]; the tide of emigration will commence - I fear to flow outward - once more, and we shall begin to decline and retrograde instead of advancing, as I had fondly hoped we should do. And what I say of my own State I may justly say of the other southern States. But, sir, I do not press that view of the subject. I know that here [in Congress] we are too weak to resist or to defend ourselves; those who sympathize with our wrongs are too weak to help us; those who are strong enough to help us do not sympathize with our wrongs, or whatever we may suffer under it. No, sir this bill will pass. And let it pass into the statute-book; let it pass into history, that we may know how it is that the South has been dealt with when New England and Pennsylvania held the power to deal with her interests." - Congressional Globe, 36-2, p 898-905
As for tariffs or anything other than slavery being a reason for secession, it is all revisionist nonsense.
If that is so, then why are tariffs so frequently mentioned as a grievance in secession era speeches, newspaper articles, pamphlets, and writings?
Each of the eleven Confederate states published declarations of secession. Every word of every one of them is about slavery and the need to keep blacks people down.
That too is simply not so. Only four states published declarations reflecting anything of the sort and those four documents, though heavily dominated by slavery, do NOT devote every word to it. Georgia's, for example, discusses the tariff issue across several paragraphs. Beyond those four declarations, none of the other secession convention measures even mentions slavery as a cause. That includes 11 secession ordinances, 2 ordinances from rump conventions in Missouri and Kentucky, and a territorial ordinance in Arizona. The Cherokee nation also issued a secession declaration after the war began, and it mentioned slavery only briefly in a long list of grievances, most of which involved violations of constitutional rights and liberties by the northern government.