The main theme of the debates was slavery, especially the issue of slavery expansion into the territories. It was Douglas' Kansas-Nebraska Act that repealed the Missouri Compromise ban on slavery in the territories of Kansas and Nebraska, and replaced it with the doctrine of Popular Sovereignty, which meant that the people of a territory could decide for themselves whether to allow slavery. Lincoln said that Popular Sovereignty would nationalize and perpetuate slavery.[6][7] Douglas argued that both Whigs and Democrats believed in popular sovereignty, and that the Compromise of 1850 was an example of this. Lincoln said that the national policy was to limit the spread of slavery starting with the Northwest Ordinance of 1787, which banned slavery from a large part of the modern-day Midwest. Lincoln pointed out that the Compromise of 1850 was just that, a compromise. It allowed the territories of Utah and New Mexico to decide for or against slavery, but it also allowed the admission of California as a free state, reduced the size of the slave state of Texas by adjusting the boundary, and ended the slave trade (but not slavery itself) in the District of Columbia. In return, the South got a stronger fugitive slave law than the version mentioned in the Constitution. Whereas Douglas said that the Compromise of 1850 replaced the Missouri Compromise ban on slavery in the Louisiana Purchase territory north and west of the state of Missouri, Lincoln said that this was not true, and that the compromises that allowed the territories of Utah and New Mexico to decide on slavery applied only to the specific issues decided as part of the Compromise of 1850.
There were partisan remarks, such as Douglas' accusations that members of the "Black Republican" party such as Lincoln were abolitionists. Douglas cited as proof Lincoln's House Divided speech in which he said, " I believe this government cannot endure permanently half Slave and half Free."
Douglas also charged Lincoln with opposing the Dred Scott decision because "it deprives the ***** of the rights and privileges of citizenship." Lincoln responded that "the next Dred Scott decision" could allow slavery to spread into free states. Douglas accused Lincoln of wanting to overthrow state laws that excluded blacks from states such as Illinois that were popular with the northern Democrats, and said, "I believe this Government was made on the white basis," and said that states (including Illinois) should be allowed to exclude "inferior races." Lincoln did not argue for complete social equality. However, he did say Douglas ignored the basic humanity of blacks, and that slaves did have an equal right to liberty.
For more info, see my source page