You are spreading half truths. The Morrill Act passed the House well before even one single southerner had left congress. The vote fell precisely on sectional lines, perhaps more than any other vote before the war. Only 1 southerner favored the thing, with otherwise unanimous southern opposition. The situation was opposite with near unanimous northern support, be they democrat or republican.
By 1861 the situation was clear. The incoming president wanted the tariff and had indicated he was going to push for it heavily. The senate was sectionally split almost down the line, but with the North lining up behind the thing practically unanimously a vote to pass it was definately in reach no matter what southern states had left or not.
My research says the Morril Act passed congress in 1862, not 1861 or 1860 as one of your posts said. There was an "income tax" bill that passed in 1861 that was never enforced and a new "graduated" income tax bill was passed in 1863.