Earliest Reasonable Abolition of Slavery? (United States)

Title says it all really.

I'm wondering just how early the United States can abolish slavery. Since this could possibly be simply solved with the British abolishing slavery pre-Revolution, though maybe that would be a stretch for the British to do in any case.

For this, let's assume a PoD of post-1776?

Thoughts?
 
If you have some of the southern states stay with the UK then you could have it abolished it right out of the gate. Perhaps the US gets Canada but loses the south? That would make an interesting timeline.
 
Or you could have the cotton gin never invented and a bole weevil eats the cotton crops, forcing slaveowners to free their slaves, making a defacto slave free nation?
 
Link please?

I think this is the TL in question.

I wonder if gradual emancipation could have been worked into the constitution? Like a graduated phase out all the way up to the 1830s? I mean, OTL we put a sunset on the importation of new slaves. What would it take to have made the constitution put a sunset clause on the whole institution?
 
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