What does Lincoln predict will happen to the country if this issue is not rectified, or corrected?

Lincoln predicts that the country as a whole will not survive if the issue of slavery is not rectified.

"A house divided against itself cannot stand."

I believe this government cannot endure, permanently half slave and half free.

I do not expect the Union to be dissolved - I do not expect the house to fall - but I do expect it will cease to be divided.

It will become all one thing, or all the other.