William W. Freeling writes about the importance of the African Slave trade issue on "The Road to Disunion"

One reason great civil war historians, including James McPherson, Amanda Foreman, Howard Jones and Harold Holzer have been so receptive to Our Man in Charleston is because the question of reopening the African slave trade is one that has been too long neglected. William W. Freeling explained the issue well in one chapter of The Road to Disunion Vol. II Secessionists Triumphant 1854-1861, and in this particularly useful note:

The movement to reopen the African slave trade almost always receives short shrift in accounts of the coming of the Civil War. The reason: The radicalism never captured anything close to a southern majority and thus allegedly must be considered an

antebellum sideshow. But by that reasoning, secessionism, also never commanding a majority until Lincoln “coerced” the disunionists, also must be considered a sideshow. The point is that a disunionist minority ultimately made majoritarian history (as minorities often do). While anti-secessionists sometimes wished to reopen the African slave trade, the movement was primarily the secessionists’, as was nothing else. (Caribbean expansionism, for example, was most often seen as an alternative to disunion.) Thus the reopening campaign offers the best window into the (minority) mentality that would ultimately make a revolution. Something so analytically valuable deserves central consideration.

The best book on the reopeners’ movement remains Ronald Takaki’s excellent A Pro-Slavery Crusade: The Agitation to Reopen the African Slave Trade (New York, 1971). 

Freehling, William W. (2007-03-09). The Road to Disunion: Volume II: Secessionists Triumphant, 1854-1861: 2 (p. 555). Oxford University Press. Kindle Edition.