Final answer:
Lincoln opposed the Kansas-Nebraska Act because he believed that the nation could not survive as part slave and part free, and saw the act as a step towards nationalizing slavery.
Step-by-step explanation:
Abraham Lincoln voiced his opposition to slavery in the context of the Kansas-Nebraska Act because he believed in preventing the spread of slavery and ultimately putting it on the path to eventual extinction. Lincoln, in his speech on the Kansas-Nebraska Act, explicitly stated that the government cannot endure permanently divided as half slave and half free. This act, which Stephen A.
Douglas introduced, effectively repealed the Missouri Compromise line, opened both territories to the possibility of slavery, and was interpreted by many to be a step towards nationalizing slavery, which Lincoln adamantly opposed.
Additionally, Lincoln's famous quote, "A house divided against itself cannot stand," encapsulates his view that the nation could not exist in perpetuity as a mix of slave and free states; it would ultimately have to become one or the other. His concerns were prophetic as the act led to increased tensions that pushed the nation closer to the Civil War.