You can see very clearly how vulnerable the entire shipping empire of the North was upon secession, and the fear these people would have.
These people with whom we are arguing simply do not get it. Economic independence of the South from the North very badly damages the Northern economy in many disparate ways that are not obvious to people who are not accustomed to thinking in these terms, but the Businessmen of New York/New England area were not fools. They knew very well that a major source of economic power was shifting out of their grasp, and it would spell ruin or severe attenuation of their finances if the South remained independent.
An additional 40% of direct cash flow to the South, (cutting out New York middlemen) plus the added economic impact of lower prices from the reduced tariff on imports would have initiated boom conditions in all areas of the South from which such profits could be made.
This would quickly result in investments in businesses such as textiles and machine factories, because the capital to make such investments would be there.
Not only would the North lose the immediate revenues from the trade, they would be facing an arising competition against their own existing industries, such as steel and textiles.
Southern independence was a financial nightmare for these men, and they were intelligent enough to realize that Southern independence would spell economic doom if it was allowed to continue.
Up and until December of 1860, the political disagreements and Congressional battles were about the balance of power between the agricultural South, and the Mercantilist North and Midwest.
Within six weeks, and due to the advent of a new tariff and a new political platform, the argument was now between the establishment Union and a progressive South on the verge of economic explosion.
The issue of slavery was now a non-issue for these people.
Add to that the superiority of the delivery system of the Mississippi that would destroy the Northeastern shipping system as well as the tariff schemes, and no wonder Governors, businessmen, and Congressmen were lined up at Lincoln's door to require him to stop the South.
Those businessmen with strong economic interests in the South were all Democrats.
They hated abolition, they hated Republicans and they hated "Ape" Lincoln.
They were partners economically and politically with Southern Slaveocrats.
When secession was first threatened these Democrat businessmen favored granting whatever concessions on slavery secessionists might want.
Once secessions were declared, these Democrat businessmen favored the most congenial relations possible with the Confederacy.
By sharp contrast, only some Republicans demanded the President and Congress take a hard line with secessionists, grant them no demands, don't let them seize Federal properties in the South.
And when the Confederacy launched war at Fort Sumter, those Republicans demanded rebellion must be defeated.
but even amongst Republicans, the majority had counselled surrender of Fort Sumter.
So Lincoln's choice of action there was his own, not Republicans generally and certainly not Northern business Democrats just recently divorced from their Southern Democrat political spouses.