Missouri is set to put an intellectually disabled man to death, after Pope Francis and politicians urged the governor to halt the execution

• Ernest Lee Johnson is set to be executed on Tuesday for killing three people in a 1994 robbery.
• Pope Francis is among those who have asked Missouri's governor not to go through with the execution.
• Johnson's lawyer says his client's intellectual disability makes execution unconstitutional.

Missouri is set to execute a murderer on Tuesday, despite high-profile pleas for mercy from Pope Francis and others who say the man's intellectual disability makes execution unconscionable.

Ernest Lee Johnson is set to be executed at 6 p.m. local time at the state prison in Bonne Terre, Missouri, according to local outlet KMIZ.

Johnson was sentenced to death for using a hammer to kill three convenience store workers during a robbery in 1994, The New York Times reported.

Due to his mother's addiction issues, Johnson was born with fetal alcohol syndrome, which the Centers for Disease Control and Prevention says can lead to issues with learning, memory, communication, and getting along with others. Johnson also lost about 20% of his brain tissue during a surgery in 2008 to remove a tumor, which may have impacted his mental capacity more, Vice reported.

Johnson's public defender, Jeremy Weis, has tried to help him evade the death penalty, telling the Associated Press that his client "meets all statutory and clinical definitions of an intellecutal disability" and has an IQ of 67-77, which is well below the average of 90.

A 2002 US Supreme Court decision found that executing intellectually disabled people was a violation of the Eighth Amendment's ban on cruel and unusual punishment.

But in August, the Missouri Supreme Court denied a petition to stop Johnson's execution based on that landmark case, according to Vice. The court said there was no official cut-off to diagnose someone as intellectually disabled based on IQ, and that Johnson's IQ tests from decades ago indicate that he may not be mentally disabled at all.

Pope Francis, US Reps. Cori Bush and Emanuel Cleaver II (both Missouri Democrats), and former Missouri Gov. Bob Holden are among those who petitioned Gov. Mike Parson, a Republican, not to execute Johnson.

Francis made his appeal in a letter through the Vatican's ambassador to the US, Archbishop Christophe Pierre.

According to Vatican News, the pope's appeal was not "based solely upon Mr. Johnsons' doubtful intellectual capacity."

"His Holiness wishes to place before you the simple fact of Mr. Johnson's humanity and the sacredness of all human life," Pierre wrote.

Nevertheless, Parson said in a statement on Monday that the execution would go on as scheduled Tuesday evening.

Johnson's lawyer has filed a last-ditch appeal with the Supreme Court, which hasn't yet responded to the request.