The US Supreme Court had earlier rejected a final appeal in the case of Warren Hill, a 52-year-old African American with a reported IQ of 70, who has spent the last 21 years on death row for killing a fellow inmate.
His planned execution has drawn protests from rights activists and mental health advocates calling the sentence a miscarriage of justice.
"Rightful liberty is unobstructed action according to our will within limits drawn around us by the equal rights of others. I do not add 'within the limits of the law' because law is often but the tyrant's will, and always so when it violates the rights of the individual." Thomas Jefferson