There are several kinds of evidence that the tariff caused the war.
Several of Lincoln's loyal advisors, who were in a position to know the facts, reported to him in writing that the tariff, not slavery, was the cause of southern secession. They declared that southerners left, not out of any fear for their slave property, but because they expected great prosperity in their new free-trade confederacy. Southerners wanted to get out from under the protective tariff regime they knew was inevitable.
There was an enormous amount of money at stake. The country could either have northern manufacturing prosperity under a protective tariff regime or it could have southern agricultural prosperity due to high export prices of cotton, tobacco and rice under a free-trade regime. It could not possibly have both at the same time. Knowing this relationship is an absolute necessity for understanding the cause of the Civil War. Wealth for the one, obtained by means of government, must come from the pockets of the other--in multiple. The financial incentives on both sides were large.
High tariffs devastated southern agricultural revenues. Tariff funds spent for internal improvements caused multiplied losses in southern income.
Congressional votes on the tariff over a period of more than forty years show a sharp sectional division. The balance of power was a very narrow one. Victory in the vote was precarious. The defeated side agonized and asked, "What if?"
The election of Lincoln to the presidency, the success of the Republican political party in the North in the 1860 Congressional elections and adjustments in Congressional representation necessitated by the 1860 census meant that the balance of power to control tariff legislation was shifting permanently from South to North. Increases in the tariff were a certainty. Southern economic distress lay ahead.
The political platform of the Republican party was constructed solely with a view to raising the tariff for protection of domestic industry, to provide funds for internal improvement of rivers and harbors, canals and roads, to give free land to settlers in the West and to provide for the construction of a Pacific Railroad. All else, including restricting the westward migration of southern planters and preserving slavery in the South, was maneuvering for a shift in the balance of poltical power to raise the tariff.
Supporters of the Republican party were men who had financial interests in raising the tariff for protection of domestic industry or to provide funds for internal improvement of rivers and harbors, canals and roads. Others had financial interests in dispensing free land to settlers in the West or construction of a Pacific railroad.
This highly credible evidence should become familiar to anyone interested in understanding the cause of the war.