James M. McPherson, "Lincoln, Abraham," American National Biography Online, February 2000, http://www.anb.org/articles/04/04-00631.html.
In retrospect, Lincoln was the real winner of the Lincoln-Douglas debates. His famous question at Freeport forced Douglas to enunciate the "Freeport Doctrine" that settlers could keep slavery out of a territory despite the Dred Scott decision by refusing to enact and enforce a local slave code. The Freeport Doctrine further alienated Douglas from southern Democrats and kindled their demand for a federal slave code in the territories. This issue split the Democratic party in 1860, virtually assuring the election of a Republican president. The national visibility achieved by Lincoln in the debates caused his name to be increasingly mentioned as the possible Republican nominee.