It was part of the equation right from the beginning... the fact that Lincoln articulated it later, differently or possibly for different reasons that people in the South, is not the issue, historically.
The South was an agrarian economy and they were only successful and prosperous at it because they held slaves.... their way of life and consequently their politics as well was tied to slavery.
For THEM, it was a central issue right up front and only became so for everyone else, later.
That’s the way I always understood it.
I believe all those things to be true. There are nuances associated with those views that make it a complicated discussion at best. As I told someone the other day, it wasn’t just that Lincoln got elected, it was that the entire balance of power in Congress had swung the other way. Slave states knew their days were numbered one way or the other.
They made a bunch of wrong moves and played into Lincoln hands. At the same time, at one point northern support for the war was so low that even Lincoln probably would not have been able to continue with it had Confederates won one more major victory. That’s what Antietam was all about.