Not sure how plausible this is. There's a reason it died OTL. When was this proposed and by whom?
But assuming it somehow happens...
This puts southern OH, IN and IL in Slave turf along with Kansas (now not a violation of the Missouri Comp), Colorado, Utah, and Nevada...and Cali, though that was within the "slave" zone OTL and fought hard to become a Free state anyway.
Now, without advanced plows or windmills Kan. and Col. are ill-suited to plantation ag., and Utah & Nevada totally ill-suited without serious irrigation efforts, so Slavery is never going to become as major as the SE and may be slave in name only. The Mormons will resist slavery in Utah as well.
The BIG deal now is OH/IN/IL and that rich Ohio R. valley. No Cotton or Tobacco with the climate, though.
It might just mean earlier conflict Bleeding Kansas style, but ironically if the Midwest Freesoilers prevail as OTL (likely IMO with the NWO still in effect) this makes less difference than you might think since I'd assume Utah and Cali stay free anyway and the rest is ill-suited to plantations and may be unlikely to secede even if slave states.