There are several kinds of evidence that the tariff was the sole cause of the anti-slavery controversy.
First, anti-slavery agitation arose at the hands of tariff-interested men.
Second, the anti-slavery controversy arose immediately following the defeat of tariff measures in Congress.
Third, the delicate balance of power in the Senate and House of representatives on the tariff issue made anti-slavery agitation a very practical political tool for shifting the balance of power to control the tariff. Because slave state members generally voted against high tariffs, restriction of slave owners from just one new state could make a difference.
Fourth, private letters from key northern anti-slavery politicians described a secret conspiracy to agitate against slavery and explicitly connected anti-slavery with the balance of power and the tariff in particular. Private letters of other knowledgeable observers recognized that the anti-slavery crusade was designed to shift the balance of power.
Fifth, the complete absence from the ranks of anti-slavery agitators of persons who were not tariff men or paid by them, suggests that there was no upwelling of genuine humanitarian sentiment in the general population.
Sixth, there was web of financial ties driving the anti-slavery crusade. Those with the most at stake on the tariff contributed the most to the anti-slavery crusade.
Seventh, some southerners explicitly accused the northerners of conducting a crusade in the name of liberty but with purpose of plunder.
The accumulation of the large body of evidence leaves little doubt that the political heat of the slavery controversy arose solely from the schemes of tariff men.
The leaders of the anti-slavery agitation were exactly the same men who were most interested in the tariff. The correlation between the leaders of the anti-slavery agitation and tariff interests is not just suspiciously close. It is precisely perfect. The leader of the pro-tariff agitation was the same man who led the anti-slavery agitation.
The margin of power in Congress on the tariff issue was narrow--usually only one or two votes in the U.S. Senate. After the passage of the Tariff of 1846 which lowered tariff rates, Horace Greeley, in a short, one-paragraph article following his editorial condemning the new tariff law in the July 31, 1846 New-York Tribune, made sure his readers were aware of the narrow balance of power in Congress on the tariff issue:
Coincidence.-It is mentioned as a remarkable coincidence that one vote carried the Tariff of 1824; one vote the Tariff of 1828; one vote in each House carried the Tariff of 1842; and by one vote in the Senate the Tariff of 1846 has become a law.
That nearly even balance of power made it possible that only a slight shift could tilt the nation in favor of protective tariffs. If southern planters could be restricted from moving into new territories that would later become states, the balance of power in the Senate would eventually shift to the North. That made anti-slavery a very practical strategy for shifting the balance of power on tariff votes. It ultimately bore fruit.
Political anti-slavery agitation arose only at the precise times of tariff frustration. Northern men carefully tabulated the votes on tariff and spending legislation in Congress. When it became apparent to them that the financial benefits they sought by means of government were narrowly thwarted by southern representation, they schemed and began actively working to shift the balance.
Most of the slaves were located in the southern agricultural states that were opposed to protective tariffs. Because of the convenient geographic division, those states could be easily isolated and made a target of propaganda for demonizing tariff-resisting slave owners. "Demonization" propaganda is an old technique, tried and true. Some of those leading the anti-slavery conspiracy had also used demonization techniques on other groups for political gain.
At least four men who were or would become U.S. presidents--Jefferson, Madison, Monroe and John Quincy Adams--recorded their observations at the beginning that the slavery agitation was motivated solely by desire for the balance of power.
Other writings have survived that describe the conspiracy in unmistakable terms giving its motive, objective and mode of operation. They identify the men who would be privy to the secret and give explicit instructions to implement the conspiracy in the newspapers of particular political orientations.
The Republican political party that came to power in 1861 had taken two separate positions in its political platform with respect to slavery, each with a shrewd political purpose designed to restrict southern political power.
First, the party favored the preservation of slavery in the states in which it already existed. Preservation of slavery in those states meant that Negro slaves would still have only the Constitutionally mandated three-fifths representation in the U.S. House of Representatives and not the five-fifths representation of freemen. Full representation would give the South more members in the House of Representatives. That would threaten northern dominance in the House on the tariff issue.
Second, the Republicans opposed westward expansion of slavery into the new territories. If southern planters were allowed to take their slaves with them to the fertile new lands in the West, they might take political control there when the territory became a state. Because the margin of power was so narrow, two new senators, voting with the South against the tariff, were the stuff of nightmares for the northern tariff men.
Republicans were for or against slavery only as it affected the balance of political power in their favor to control the tariff.
Some of the most popular evidence offered by American historians to show that concern about slavery underlay the sectional enmity fails under closer scrutiny. Quotations of southern leaders have been "doctored" by excising key language. With excisions restored, the quotations not only fail to show the institution of slavery as a cause, but are actually strong evidence that the tariff was the cause.
Southerners, too, abused the slavery issue. At first, they countered northern propaganda by treating slavery as a positive good--the best situation for the African Negro. They intended to fight off efforts to restrict slavery from westward expansion.
Finally, at secession time, they demonized Republicans as abolitionists threatening slave property. Southern leaders clearly knew better, as Hosea informed Lincoln, but they found the slavery issue so attractive as an inflammatory propaganda tool that they used it liberally to "fire the southern heart" to bring about secession to get free trade. Lincoln, who afterwards called the slavery issue in the South an "artificial crisis," clearly agreed with Hosea.
Historians asserting slavery-as-cause cite the South Carolina Declaration of Immediate Causes as proof. That document and its companion Address are compared to show the tariff as cause.