In a letter to J. Scripps, Lincoln seeks to clarify his ‘A House Divided Speech.' For a further response by Lincoln to his speech, see his 2/14/60 letter. LL 308-9
6/23/1858 I have declared a thousand times, and now repeat that, in my opinion, neither the General Government, nor any other power outside of the slave states, can constitutionally or rightfully interfere with slaves or slavery where it already exists.
I believe whenever the effort to spread slavery into the new territories, by whatever means, and into the free states themselves, by Supreme Court decisions, shall be fairly headed off, the institution [slavery] will then be in course of ultimate extinction... FT 52