A death-row inmate denied clemency Monday is asking the nation's highest court to stop his execution on grounds he is mentally disabled.

Warren Lee Hill is scheduled to be put to death Wednesday by lethal injection for beating fellow prison inmate Joseph Handspike to death with a nail-studded board. At the time, Hill was serving a life sentence for killing his 18-year-old girlfriend.

Hill's lawyer, Brian Kammer, said he was "horrified and outraged" by the parole board's decision to deny clemency, noting that a state judge previously found Hill to be mentally disabled.

Lawyers for Hill are now asking the U.S. Supreme Court to stay the execution. The high court declined in June to hear Hill's latest appeal, but his lawyers asked the court to reconsider. Hill is asking the court to postpone the execution until it rules on that request.

The case is "close to becoming a miscarriage of justice," Hill's motion said.

If that fails, Hill will file new appeals in the state and federal courts, said Kammer.

Hill's problem is that the judge found him mentally disabled by a preponderance of the evidence — or more likely than not. Georgia's law, the only one of its kind and the strictest in the nation, requires proof beyond a reasonable doubt.

"The Board is making the same mistake it made in denying clemency for another mentally retarded inmate, Jerome Bowden, in 1986," Kammer said. "This shameful decision violates Georgia's and our nation's moral values and renders meaningless state and federal constitutional protections against wrongful execution of persons with mental retardation."

In a statement last week, the state Attorney General's Office said Hill had failed to prove he was mentally disabled.

In response to the public backlash to the execution of Bowden, who had an IQ of about 65, Georgia became the first state in the country to ban the execution of the mentally disabled. But the 1988 state law also requires capital defendants to prove their disability by clearing the stringent legal threshold.

Hill's case attracted pleas for mercy from former President Jimmy Carter and his wife Rosalynn as well as state and national groups for the developmentally disabled.

Rita Young, director of public policy for All About Developmental Disabilities in Decatur, said her group believes Hill should be allowed to spend the rest of his life in prison without the possibility of parole. "We are shocked and deeply disappointed in the board's decision to execute a man with an intellectual disability," she said.