The passage of the Morrill Tariff was a consequence of secession, not a cause.
It passed the Senate by 25 to 14, but only after the seven initial CSA states had seceded. Until then, Democrats had controlled the Senate, and the measure didn’t even come to a floor vote. Had these states remained in the Union, presumably any vote would have been 28 to 25 against.
It is also interesting that the measure was signed by Buchanan two days before leaving office. He was, of course, a Democrat.
I and others before me have noted on these threads that a head count was made in December, 1860 that showed that the Morrill Tariff would pass when the new Senate elected in the fall of 1860 took office. If all Southern senators had stayed in Congress in early 1861, they could have stopped it briefly as you say, but the bill would pass in the new Senate even if all Southern senators stayed for the new term.
I’m going back to bed.
It passed the Senate by 25 to 14, but only after the seven initial CSA states had seceded
Strictly speaking, the seventh state to secede, Texas, did not officially secede until the specified date of March 2, 1861 (Texas Independence Day, the date of the victory over Santa Ana at San Jacinto) and after Texas voters overwhelmingly approved secession on February 23. The Senate vote you cite was on February 20, before Texas voters, the sovereign voice of the state, approved secession.
Another possible problem comes to mind if Senators from states that had already seceded stayed and blocked the passage of the tariff at least temporarily until the new Senate was sworn in. Would Northern senators have accepted the blockage of the bill by Senators whose states had already seceded. Texas Senator Wigfall stayed in the Senate after March 2 with the excuse that he was waiting for official notification from Texas that they had seceded. There were complaints on the Senate floor that he should no longer be there since his state had seceded. If he had succeeded in blocking something they really wanted (little chance of that) I suspect they would have booted him out. For all I know Wigfall might have left the Senate in March of 1861 under threat of expulsion.
It is also interesting that the measure was signed by Buchanan two days before leaving office. He was, of course, a Democrat.
He was a Democrat from Pennsylvania, a state whose industry would benefit from protective tariffs. That and the fact that he was leaving office is probably why Buchanan signed the bill. A more perverse reason might have been that he may have wanted to stick Lincoln with the tariff because of the later huge problem of loss of federal revenue that arose because of the later differences between the Northern and Southern tariffs. But I doubt he could see that far ahead or was that partisan.