Former one-term U.S. Representative Abraham Lincoln was campaigning to take Douglas's U.S. Senate seat by strongly opposing all attempts to expand the geographic area in which slavery was permitted.
Lincoln tried to force Douglas to choose between the principle of popular sovereignty proposed by the Kansas-Nebraska Act (which left the fate of slavery in a U.S. territory up to its inhabitants), and the majority decision of the United States Supreme Court in the case of Dred Scott v. Sandford, which stated that slavery could not legally be excluded from U.S. territories (since Douglas professed great respect for Supreme Court decisions, and accused the Republicans of disrespecting the court, yet this aspect of the Dred Scott decision was contrary to Douglas's views and politically unpopular in Illinois).
He heard me argue the Nebraska bill on that principle all over the State in 1854, in 1855, and in 1856, and he has no excuse for pretending to be in doubt as to my position on that question.
Those police regulations can only be established by the local legislature; and if the people are opposed to slavery, they will elect representatives to that body who will by unfriendly legislation effectually prevent the introduction of it into their midst.
These Southerners subsequently insisted on the Congressional repudiation of the Freeport Doctrine (i.e., the passage of slave codes for the territories), in order to block Douglas's presidential bid in 1860.