[BroJoeK]

from the article: "By September he told Julian Sturtevant that 'if it were not' for knowing 'that if Mr. Douglas's doctrine prevails, it will not be 15 years before Illinois itself will be a slave state...I would retire from the contest'."

I've seen where Lincoln expressed the same ideal elsewhere, so it was likely a common theme and may help explain how Northerners previously agnostic on slavery flipped to become ardent abolitionists.
So long as slavery remained in the South they were OK with it, but expansion into western territories and even more making all states effectively slave-states, that got their blood boiling.

[Tax-chick]

Yes, that’s an excellent point.

[henkster]

Mr. Lincoln and other abolitionists can thank Mr. Chief Justice Taney for the ability to make that a credible argument.