For Lincoln the looming problem was greater than Charleston and Ft Sumter. The rump Confederacy was essentially a free trade zone with dramatically lower tariffs and it was right on his doorstep. Trade that could divert would skip US ports and go to CSA ports, draining more than just the usual traffic that those ports normally received.
One of the largest problems would be the Port of New Orleans where all the interior trade headed down the Mississippi River. The USA already had had experience with this when New Orleans was controlled by Spain prior to the Louisiana Purchase. Some western states had flirted with joining Spain because of the close trade ties. They might similarly be lured into the CSA orbit for the same reason.
Yes, yes, I know, it's the argument DiogenesLamp has been making for ages.
And it's never been true when DL says it, so it's still not true when you make the same argument.
First of all, the fact is that Confederate tariff rates were not "dramatically lower" than Morrill Tariffs, but yes, they were lower -- they were the same as US pre-Morrill rates, approx. 15%.
If Congress seriously believed those were a major threat to US trade, they need simply reduce Morrill back to pre-Morrill levels.
But there's no actual data suggesting this potential problem ever materialized.
Indeed, during the Civil War, when all exports from Confederate states were blockaded, and so supposedly the Union would lose 70% (!) of the income needed to pay for imports providing Federal revenues... well, nothing of the sort happened.
Indeed, when push came to shove, Congress quickly found new sources of revenue, more than enough to pay for the 15 times increase in war-time spending versus that of 1860.
Of course, once the Confederacy started and declared war, then General Scott's long pre-existing "Anaconda Plan" went into effect, the purpose of which, as its name implies, was to strangle the Confederacy economically.
To that degree, Lincoln certainly did believe that economic factors would play a critical role in defeating the Confederate military.
And, of course, Lincoln was right about that.