That's a red herring as there were northern Democrats from states like Pennsylvania and New Jersey who favored protection. One of the bill's main co-sponsors in the Senate was Bigler from Pennsylvania - a Democrat - and it was eventually signed by President Buchanan - another Democrat from Pennsylvania.
Below is the speech from Alexander Stephens, against secession
Quoting Stephens on the tariff act in 1860 is fallacious. He was removed from the debates of Congress at that time and did not comprehend the economics of the issue as other members of the House and Senate did.
Senator Wigfall of Texas actually rebutted Stephens' calculation claims on the floor of the chamber and in specific detail:
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...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"
It should be noted that when time came to pass the Morrill Tariff, Senator Hunter of Virginia - who had delayed the bill from coming to the floor for almost a year by exerting every parliamentary tactic at his disposal - similarly remarked:
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.
The South had full representation in Congress.
That is simply a lie. The Morrill Tariff passed the House with ease in May of 1860 despite virtually unanimous opposition by every southern member. As I noted, Senator Hunter exerted every bit of parliamentary strength he could to delay the vote in the Senate until after the election hoping for the slim chance that enough votes would emerge to block it. They did not and had every single southern member stayed in the Senate and voted against the Morrill Tariff, the best case scenario they could have hoped for was a tie, in which case Vice President Hamlin would cast the deciding vote in favor.
LOL!