In an attempt to respond to Stephen Douglas' relentless race-baiting, Abraham Lincoln reaches the moral nadir of his political life. But from the depths of this disaster he starts finding an argument that he will wield with far more effectiveness in the later debates.

Sources used:

Blumenthal, Sidney: All The Powers of Earth. Simon and Schuster, 2019.
 
Burlingame, Michael, Abraham Lincoln: A Life. (Vol. 1). The John Hopkins University Press, 1995.

Collected Works of Abraham Lincoln. 1953 Edition hosted by the University of Michigan.

Davis, Rodney and Wilson, Douglas, ed. Herndon's Informants: Letters, Interviews, and Statements about Abraham Lincoln. University of Chicago Press, 1998.
 
Davis, Rodney and Wilson, Douglas, ed. Herndon on Lincoln: Letters. Knox College Lincoln Studies Center, 2016.

Davis, Rodney and Wilson, Douglas, ed., The Lincoln-Douglas Debates. Knox-CollegeLincoln Studies Center, 2008.

Donald, David Herbert, Lincoln: A Biography. Simon and Schuster, 1995.

Foner, Eric, The Fiery Trial:  Abraham Lincoln and American Slavery. W.W. Norton and Company, 2011.

Guelzo, Alan, Lincoln and Douglas: The Debates That Defined America. Simon and Schuster, 2008.