“I suspect the tariff increase would have been moderate had the southern representatives not already jumped ship. The secession started in December before Lincoln and the new Congress even took office. And according to the secessionists’ own statements, tariffs were at most a minor point.”
Absolutely not true. The oppressive tariffs were proposed before the 1860 election, and Abe said he supported them BEFORE the election. Thus, the writing was on the wall before the election, and the Southern states knew, with a Lincoln electoral victory, that its economy was destined to collapse. Lincoln signed the tariffs into law in March, 1861 upon taking office. Your comment that the tariffs were “at most a minor point” is just folly: Not only did the South rage about the tariffs, the objective chroniclers in Europse almost to a man called the American Civil War the “Tariff War.”
The heavy tariff would have never passed if the departed southerners had joined forces with the northern Democrats. And I doubt that even Republican support would have been as strong without suppression of a rebellion to finance. If you read the southern secession declarations they were dominated by the slavery issue. In their own words, tariffs were a minor irritant at most.