Final answer:
Northerners objected to the division of the Nebraska Territory as it could enable the spread of slavery, which many opposed. The Kansas-Nebraska Act's proposal for popular sovereignty and the repeal of the Missouri Compromise raised fears that this would disrupt the balance of power and was seen as succumbing to southern interests.
Step-by-step explanation:
The opposition of some northerners to the dividing of the Nebraska Territory into two separate territories—Kansas and Nebraska—stems from their concerns about the spread of slavery. The Kansas-Nebraska Act, proposed by Senator Stephen Douglas in 1854, intended to organize the territory and facilitate the completion of a national railroad. However, a controversial element was the repeal of the Missouri Compromise, which had previously limited the expansion of slavery. This repeal meant that popular sovereignty would be used to determine if Kansas and Nebraska would be slave or free states. Since this could potentially lead to the expansion of slavery, many northerners who opposed its extension saw the act as a capitulation to southern interests and a threat to the balance of power between slave and free states.
Northern resistance was further strengthened by the perception of a slave power conspiracy, which heightened tensions. The situation was exacerbated when southern senators demanded concessions as a condition for their support, leading to the explicit repeal of the Missouri Compromise. Subsequent violence in Kansas and political upheaval indicated the deepening division between the North and South, contributing to the events leading up to the Civil War.