Heart in the right place? Is that enough?

Why Georgia Gets To Thumb Its Nose At The Constitution’s Ban On Executing The ‘Mentally Retarded’

Ian Millhiser writes,

More than a decade ago, the Supreme Court held that “death is not a suitable punishment for a mentally retarded criminal.”

It has never been all that clear to me whether a punishment counts as unconstitutional only if it is both cruel and unusual.

Or is either alone prohibited by the 8th Amendment?

Excessive bail shall not be required, nor excessive fines imposed, nor cruel and unusual punishments inflicted.

I guess both, or they would have written "or" instead of "and," with the result that change of punishment practices, away from corporal toward fines and imprisonment or toward more humane methods of execution, for instance, would be banned. 

But, anyway, what does the 8^th Amendment have to do with the states?

I don't think this liberal fiction passes the test.

Would an amendment giving it a legitimate constitutional basis be passed?

I could be wrong, but I think not.