Your canard: “But there was no Morrill tariff so long as Southern Democrats dominated Congress and the Presidency.”
Wrong. It had passed the House well before secession.
The vote was on May 10, 1860; the bill passed by a vote of 105 to 64.
Then we have the 1860 elections.
The results of the election produced a reapportionment of Congress that all knew that even if one assumes that every single seceded state's senators had (a) remained and (b) voted against the Morrill act, they still would not have been able to muster enough votes to defeat it.
In the absolute best case voting scenario that could have occurred under the senate that took office in 1861, the best that the southerners could manage would be a tie vote, in which case VP Hanibal Hamlin would cast a tiebreaker in favor of the north and the tariff would pass. The southerners recognized this fact almost immediately after the 1860 elections and publicly stated so.
The Morrill bill was brought to the Senate floor for a vote on February 20, and passed 25 to 14.
The economic order of the United States was dramatically changed. The tariff took off on an upward trajectory that was far above any tariff in history.
Another Brojoke Canard Bites the Dust.
No, the fact is: so long as Deep South Congressional Representatives & Senators remained on the job, the new Morrill Tariff did not, and could not, pass.
The bill only passed after they began to resign.
Now, as to what woulda, coulda, shoulda happened if pro-Confederates had stayed in Congress, nobody knows, or can know.
Remember, not every Northerner favored higher tariffs and not every Southerner wanted lower tariffs.
Many were eminently, ahem, "persuadable" depending on the views of their own voters plus other "inducements".
So the issue really came down to: who wanted the higher or lower tariffs worse, and which side would put more, ah, "energy" into the political battle.
Finally, I'll remind you that the notorious "Tariff of Abominations" happened under President Jackson and Vice President Calhoun.
So the idea that all Southerners, even Deep South officials were forever and adamantly opposed to all tariffs is pure ex post facto pro-Confederate propaganda.