As stated in the Republican party platform of 1860:
2. That the maintenance of the principles promulgated in the Declaration of Independence and embodied in the Federal Constitution, "That all men are created equal; that they are endowed by their Creator with certain inalienable rights; that among these are life, liberty and the pursuit of happiness; that to secure these rights, governments are instituted among men, deriving their just powers from the consent of the governed," is essential to the preservation of our Republican institutions; and that the Federal Constitution, the Rights of the States, and the Union of the States must and shall be preserved.
For slaveholders, once Lincoln was elected, that provision was like a dagger pointed at them and slavery.
Why would that be? Lincoln orchestrated passage of the Corwin Amendment and endorsed it in his first inaugural address. He also offered strengthened fugitive slave laws. What he did not offer was any abatement of the Morrill Tariff.