I'm aware of the Copperheads and their desire to restore the Union "as it was" (read: with slavery), but I'm curious to know if there were any groups that were both anti-slavery and supported the idea of (for lack of a better way of saying it) "cutting the South loose."
I've never heard of or read of any such group existing. Now, you could argue that Garrisonians were of this mindset in the 1840's-1850's, what with Garrison's insistence on 'No union with slaveholders,' but aside from this disunionism I've never seen any evidence of what you've imagined existing.