Court Denies Stay Of Execution For Texas Inmate.

Robert James Campbell

 (CNN) -- Texas plans to put to death Tuesday a convicted rapist and murderer who, a neuropsychologist says, is "mildly mentally retarded," in the nation's first execution since a botched lethal injection in Oklahoma left an inmate writhing in pain before death.

Robert James Campbell's defense team is challenging, on a variety of grounds, the state's decision to execute him, including ineffective assistance of counsel, state misconduct, Texas' refusal to divulge the source of its execution drugs and the man's mental capacity.

In 2002, the U.S. Supreme Court concluded "the mentally retarded should be categorically excluded from execution."

The U.S. Fifth Circuit Court of Appeals denied motions for a stay. The motions cited the mental retardation and drug-source claims.

Texas Attorney General Greg Abbott, in a petition contesting the defense claims of mental retardation, questioned why Campbell waited until 12 years after a court had determined his mental state to raise the claims.

"Campbell's last-minute claim of mental retardation, which was previously raised and rejected in the federal and state courts does not warrant review. Campbell is not mentally retarded," according to pertinent case law, Abbott contends.

The execution of Campbell, 41, is slated for 7 p.m. ET at the Texas State Penitentiary at Huntsville, about 70 miles north of Houston. The facility, nicknamed "Walls Unit" for its red brick facades, has hosted 876 executions since 1924.