Oklahoma is set to execute Benjamin Cole, a 57-year-old man convicted of the murder of his 9-month-old daughter, on Thursday, continuing a string of more than two dozen executions.

However, in the two decades since the crime, the death row inmate’s declining mental condition – exacerbated by his childhood exposure to drugs and alcohol, substance abuse issues, and physical and sexual abuse – has deteriorated to the point where Cole is no longer competent to be executed, according to his attorneys in a clemency petition.

Their claims bring to the forefront a long-standing issue in the debate over capital punishment: how it should be applied to those suffering from mental illness. According to their attorneys, the issue is critical in a number of inmates’ cases, as Oklahoma officials plan to carry out 25 executions through 2024, a spree that critics have also condemned given the state’s history of botched lethal injections.

The US Supreme Court denied Cole’s request for a stay of execution on Wednesday, after the state parole board declined to recommend clemency last month. Meanwhile, the inmate’s lawyers have asked a state appeals court to order the warden to refer his case to the district attorney for review in order to initiate a competency hearing.

The facts of Cole’s case obligate the state to spare his life, his attorneys told parole board members in recent months, though the arguments failed. They cited “evolving standards of decency,” such as public polling showing opposition to executions of the mentally ill.

The execution of the severely mentally ill was ruled unconstitutional by the US Supreme Court in 1986, with Justice Thurgood Marshall writing, “It is no less abhorrent today than it has been for centuries to exact in penance the life of one whose mental illness prevents him from comprehending the reasons for the penalty or its implications.” In Oklahoma, it is also against the law to execute someone who has been determined to be insane.

Cole, who has schizophrenia and a brain lesion associated with Parkinson’s disease, is largely “catatonic,” rarely speaking to anyone, including his own lawyers, according to his clemency petition. After years of near-total isolation in the Oklahoma State Penitentiary, he now uses a wheelchair and lives in his own “mental universe,” as one clinical psychologist described it in the clemency request, not understanding the legal proceedings surrounding his impending execution.

“Benjamin Cole’s mental illness has rendered him essentially non-functional,” his attorney, Tom Hird, said in a statement after an Oklahoma judge ruled this month that Cole was competent to be executed.

In a statement, Oklahoma Attorney General John O’Connor praised the parole board’s September vote, noting that Cole’s conviction and sentence had been upheld on appeal and dismissing concerns about Cole’s mental illness.

“Despite his attorneys’ claims that Cole is mentally ill to the point of catatonia, the fact is that Cole fully cooperated with a mental evaluation in July of this year,” said the attorney general on September 27. “The evaluator, who was not hired by Cole or the State, determined that Cole was competent to be executed and that ‘Mr. Cole does not currently demonstrate any significant, overt signs of mental illness, intellectual impairment, and/or neurocognitive impairment.’

According to the attorney general’s office, Cole was found guilty of the brutal murder of his daughter, Brianna Victoria Cole, on December 20, 2002, when her cries interrupted him while he was playing a video game.

According to a probable cause affidavit, Cole grabbed his daughter’s ankles while she was on her stomach and forced them up to her head, breaking her spine and causing her to bleed to death. Cole then returned to his video game as his daughter died, according to O’Connor.

Cole admitted to causing his daughter’s fatal injuries in a taped confession, according to his clemency petition, telling police he would “regret his actions for the rest of his life.”