What if "The right to keep slaves" had been in the US consitution along with the right to bear arms?
Pretty much same as OTL. The Federal government still has the power to do certain things like ban international slave trade (which it did), ban interstate slave trade (preventing people from selling their slaves to other states when their state eventually becomes a free state), heavy taxes, etc. Basically make keeping slavery completely worthless.
Think of how slavery was ended OTL.
1) States giving it up on their own (the North).
2) As a military action (the majority of the CSA).
3) As a constitutional amendment (the rest of the CSA, border states)
First two won't be prevented by the altered constitution. The third removes that bit of the constitution, ending the divergence (outside of any butterflies).
If you mean guaranteeing slavery
everywhere, making it illegal for states to ban slavery, then it's ASB. Many of the politicians at the time, even the ones that had slaves, felt it was a necessary evil. Something they'd get rid of if they could, but didn't want to risk the economic ruin (either to their person for freeing their own slaves, or the nation for complete abolition).
Before you say it's totally and utterly ASB, Washington and a president or two after him owned slaves.
Quite a few more than one or two, actually! IIRC, JQA was the first president to not have any slaves.
Eh. Depends on how you define "the North." The border states weren't really northern in outlook. Either west/midwest in the case of Missouri, Kentucky, and WV, or south in the case of Maryland and Delaware.