How did Stephen Douglas respond to Abraham Lincoln's belief that slavery had to halt for the United States to prosper in the Lincoln-Douglas Debates?
Essentially, Douglas remained fairly consistent in applying his argument for popular sovereignty to the debate over the expansion of slavery. He engaged in the time-honored (and still current) strategy of portraying Lincoln as a radical, associating him with "Black Republicans," an epithet used to describe politicians who supported abolition. In arguing for the principle of popular sovereignty, he asserted the "Freeport Doctrine" in response to Lincoln's question about how a state or territory could keep slavery out in light of the Dred Scott decision:
...[I]t matters not what way the Supreme Court may hereafter decide as to the abstract question whether slavery may or may not go into a Territory under the Constitution, the people have the lawful means to introduce it or exclude it as they please, for the reason that slavery cannot exist a day or an hour anywhere, unless it is supported by local police regulations.
The subtext for all this is that both men, particularly Douglas, had an eye on the 1860 presidential race. As such, each sought to force the other to take unpopular positions. If Douglas could portray Lincoln as a radical, Lincoln could force Douglas to essentially offer a way around the Dred Scott decision, in the process taking a position that would be extremely unpopular with Southern states.