(CNN) -- Warren Lee Hill, whose defenders say he is mentally disabled, is scheduled to die by lethal injection Tuesday for the 1990 killing of Joseph Handspike.
Handspike was another inmate in a Georgia state prison.
Hill was convicted of beating Handspike to death with a nail-studded board while already serving a life sentence in the 1985 killing of his girlfriend, Myra Wright.
Hill's lawyers have asked the U.S. Supreme Court to halt the execution, arguing that his IQ of 70 means he should be spared under the 2002 decision that barred the execution of the mentally retarded. But a string of state courts has said Hill doesn't qualify under Georgia law, which requires inmates to prove mental impairment "beyond a reasonable doubt."
Georgia is the only state with the reasonable-doubt standard, which Hill's lawyers call "a virtually insurmountable barrier" that flies in the face of the justices' 2002 decision.
"The U.S. Supreme Court says we don't put mentally retarded people to death, but we'll let the states determine who's retarded and who's not," CNN legal analyst Paul Callan said Monday.
Handspike's family has called for the execution to be called off, as have former classmates and school officials. The Georgia Council on Developmental Disabilities has weighed in against the execution, stating, "No other state risks the lives of those with developmental disabilities to this extreme."
Three doctors who examined Hill for the state "have now revised their opinions and find that Mr. Hill does meet the criteria for mental retardation," his lawyers argued in court papers.
But lawyers for the state argued that Hill had served in the Navy, held a job and managed his money before Wright's killing -- signs that while he had a low IQ, he didn't necessarily meet the legal standard for retardation.
Hill had been scheduled for execution in July, but the state Supreme Court halted the execution on procedural grounds. It rejected a plea to spare him due to mental retardation, and the state Board of Pardons and Paroles has denied his bid for clemency.
The execution is scheduled for 7 p.m. at a state prison in Jackson, about 45 miles south of Atlanta.
Hill refused any special last meal, the state Department of Corrections said. Instead, prison officials said, he'll be offered the institution's meal tray of macaroni and cheese, baked beans, mixed vegetables, stir-fry vegetables, cornbread, cookies and iced tea.
CNN's Dave Alsup and Bill Mears contributed to this report.