This week, the state of Texas intends to execute Scott Panetti, who in 1992 shot dead his mother-in-law and father-in-law in front of his estranged wife and their three-year-old daughter. That Mr Panetti, now 56, committed the murders has never been in doubt; he admitted as much at his trial in 1995, when he defended himself while dressed in a purple cowboy outfit and attempted to call more than 200 witnesses, including John F Kennedy and Jesus Christ.

Long before it became clear from his courtroom antics, Mr Panetti had been diagnosed as severely mentally ill, which is why his impending lethal injection – due to be carried out on Wednesday – is opposed by not only his lawyers and a familiar collection of human rights groups, but also by an alliance of conservatives.

Among those protesting against Mr Panetti’s death sentence are more than 50 leading evangelical Christians, seven Methodist bishops, 10 Texas state politicians and the libertarian former presidential candidate Ron Paul. Mr Paul, a former Republican congressman who once backed the death penalty, wrote last month to Rick Perry, the Texas Governor, to appeal for clemency in the Panetti case. It is thought to be the first time he has publicly opposed an execution.

The state’s former Democrat Governor, Mark White, said: “I know very well that in so many instances, there are incredibly close and difficult calls that have to be made to either allow or prohibit the death penalty from being carried out. But Scott Panetti’s plea for clemency is no such case. He is a severely mentally ill man. His trial was a sham. And executing Panetti would say far more about us than it would about the man we are attempting to kill.”

Mr Panetti was diagnosed as paranoid schizophrenic by army doctors in 1978. Several years later, he buried all of the furniture in his home, believing the Devil was hiding in it. He was hospitalised more than a dozen times between his first diagnosis and 1992, when his wife, Sonja, obtained a restraining order and took their daughter to live with her parents, Joe and Amanda Alvarado, in the Texas Hill Country town of Fredericksburg. She reportedly tried to have her husband committed, and even took his guns to the police, but they refused to confiscate the weapons and instead returned them to their owner.

Days later, Mr Panetti donned a camouflage uniform, shaved his head and went to his in-laws’ home, where he shot the couple dead, showering his wife and child in their blood. Later that afternoon, after washing and changing into a suit, he gave himself up.

At trial, Mr Panetti claimed he was ordered to carry out the killings by Sergeant Ranahan Iron Horse, an auditory hallucination whom he called “Sarge”. The court, which could have stepped in and compelled him to hire a lawyer, instead allowed Mr Panetti to continue defending himself. On death row, he reportedly suffers from the delusion that Satan planned his execution to prevent him preaching Christianity to other inmates. He also claims the prison dentist implanted a listening device in his tooth, and that pop star Selena Gomez is his daughter.

It is seven years since Mr Panetti’s mental competence was last evaluated. In 2007, the US Supreme Court considered his case, and in its decision said a prisoner who lacked “rational understanding” of why they were being executed should not be put to death. Yet the case was sent back to a lower federal court, which last year concluded that Mr Panetti was competent and that his lethal injection could proceed.

The state of Texas argues that Mr Panetti has exaggerated his condition, but Kathryn Kase, one of his lawyers, told the Associated Press news agency: “He cannot appreciate why Texas seeks to execute him. You have to have a rational as well as factual understanding of why you are being executed. In Mr Panetti’s case, his understanding is the state wants to prevent him from preaching the Gospel on death row and saving their souls. And clearly that’s not factual or rational.”

Texas is responsible for almost 40 per cent of executions carried out in the US since 1977, and Mr Perry has overseen more executions that any other US governor in history. Although he cannot commute Mr Panetti’s sentence without the recommendation of a state pardons board, he can grant a 30-day stay of execution – time for Mr Panetti’s lawyers to organise a new mental evaluation.

The unlikely coalition of conservatives against the death penalty has found in Mr Panetti’s case a cause célèbre. Last year, Mr Paul publicly endorsed the campaign group Conservatives Concerned About the Death Penalty, saying he believed capital punishment was “inconsistent with libertarianism and traditional conservatism”.

Some libertarians, including Mr Paul, oppose the death penalty because they distrust government’s ability to apply it fairly and effectively. Fiscal conservatives are swayed by the argument that the legal costs of executions far outweigh those of life imprisonment. Many religious conservatives have turned against capital punishment in the context of their existing opposition to abortion.