Pleas for clemency on behalf of Ernest Lee Johnson, who was convicted of a 1994 murder, are growing more frantic ahead of his scheduled execution by lethal injection in Missouri on Tuesday after the Pope and two members of the US Congress issued calls for the sentence to be set aside.

In a statement last week, Pope Francis requested clemency for Johnson in a letter to Missouri governor Michael Parson. The letter did not deny that “grave crimes such as his deserve grave punishment” but called on Parson to consider “the simple fact of Mr Johnson’s humanity and the sacredness of all human life.” If carried out it will be the first execution by Missouri since May 2020.

The Pope’s call for clemency has been joined by two of Missouri’s Democratic members of Congress, Cori Bush and Emanuel Cleaver II, who petitioned the governor to halt the execution.

Bush and Cleaver, both members of the Congressional Black Caucus, urged the governor to acknowledge “the moral depravity of executions” and argued that Johnson’s execution perpetuates the same cycles of trauma and violence against Black people as “slavery and lynching did before it”.

“The fact of the matter is that these death sentences are not about justice. They are about who has institutional power and who doesn’t,” they wrote. “Like slavery and lynching did before it, the death penalty perpetuates cycles of trauma, violence and state-sanctioned murder in Black and brown communities.”

Advocates for the condemned man, including Bush and Cleaver, say that he has had developmental delays since birth, when he was born with fetal alcohol syndrome to a mother who battled addiction. Johnson, 61, has also had an operation to take out a tumor that removed as much as 20% of his brain tissue, the AP reported, which advocates say has further reduced his intellectual capacity.

Johnson’s public defender, Jeremy Weis, has said that Johnson has scored between 67 and 77 in IQ tests and “meets all statutory and clinical definitions” of intellectual disability. Missouri law broadly defines intellectual disability as “substantial limitations in general functioning.”

But last month, the Missouri supreme court ruled that Johnson is not, as he claims, intellectually disabled and denied his request for execution by firing squad based on his claim that death by lethal injection would cause severe pain.

Johnson’s death sentence stems from February 1994 when he walked into a general store near his home in northeast Columbia and bludgeoned, stabbed, and shot three employees, Mary Bratcher, 46; Mable Scruggs, 57; and Fred Jones, 58, before hiding their bodies in a walk-in cooler and robbing the store for drug money.

Johnson’s scheduled execution has highlighted racial and social inequities in the application of justice. Issuing a call for clemency, the Kansas City Star’s editorial board noted that Parson had swiftly pardoned Mark and Patricia McCloskey, a white St Louis couple who plead guilty to assault after waving weapons at Black Lives Matter demonstrators last year.

The board criticized the governor for failing to convene a board of inquiry but said it did not “even dare to hope that the evidence that Johnson today has the awareness of a child might convince our governor to commute his sentence”.

It added: “When the state, our state, does kill this man, as it almost certainly will, it will be yet another indictment of a system so bloodthirsty that it delights in vengeance against those who don’t even know why they’re being punished.”