Final answer:
Lincoln opposed the spread of slavery, reinforcing its moral wrongness, while Douglas's Freeport Doctrine supported popular sovereignty but led to a split in the Democratic Party and weakened his chances in the presidential election.
Step-by-step explanation:
In the debates between Abraham Lincoln and Stephen Douglas, the main issue at the center was the expansion of slavery. Lincoln argued that slavery was morally wrong and believed it should not spread any further, emphasizing that the country could not survive as half slave and half free. In contrast, Douglas supported the idea of popular sovereignty, where the residents of a territory would have the right to decide for themselves whether to allow slavery by enacting local police regulations. This position was crystallized in the Freeport Doctrine during the second debate in Freeport, Illinois. Lincoln forced Douglas to reconcile this stance with the Dred Scott decision.
Douglas's Freeport Doctrine created division within the Democratic Party, especially among southern Democrats who favored the expansion of slavery. Although Douglas won the Illinois Senate seat over Lincoln, his doctrine weakened his support from Southern Democrats, contributing to his loss in the following presidential election.