The Texas Republican spearheading an extraordinary bipartisan effort to delay Wednesday’s execution of a Mexican American woman amid mounting evidence of her innocence has described the case as “the most troubling I’ve ever seen, possibly the most troubling in the history of our state”.

In an interview with the Guardian, Jeff Leach, a Republican member of the Texas House of Representatives, said that even strong supporters of the death penalty – as he once considered himself – had great concern about the rapidly approaching execution. Melissa Lucio, 52, is scheduled to be put to death by lethal injection for killing her two-year-old daughter Mariah, but new scientific evidence suggests the toddler died accidentally after a fall.

Leach said that a possibly innocent woman was “about to be – let’s not mince words – murdered by the state”. He said that the numerous missteps made in the prosecution of Lucio had given him “great pause” that was forcing him to reconsider his support of capital punishment.

“I’m not ashamed to say I believed in the death penalty for the most heinous cases. But this case is shaking my confidence in the system. It has failed Melissa and the victim of the tragedy, Mariah, at every turn,” he said.

The lawmaker had strong words to say about the agonizing wait Lucio is enduring in her final hours before her scheduled execution.

“It’s barbaric,” Leach said. “There’s no reason for it. They’ve had all the information they need for weeks – someone just has to step up and make the right decision.”

The Republican lawmaker ran through a litany of prosecution failures. He said: “If it was just the interrogation, or just one piece of new scientific evidence, or only one juror and not five who have now said they regret their decision, then we could talk. But when you have all of these things where the system has failed, then that causes great alarm.”

Leach’s comments came as Lucio’s battle to save her own life entered its final feverish hours. Her lawyers are fighting on several fronts in the hope of securing a stay of execution.

On Monday, the Texas Board of Pardons and Paroles is meeting to discuss the Lucio case, and its recommendation will then be passed to the Republican governor, Greg Abbott, for his final say. Abbott has the power to put a hold on the execution, grant Lucio clemency or let the process proceed to the death chamber.

Legal appeals have also been lodged with a local district court in Cameron county and in the Texas court of criminal appeals in Austin. A new filing on Monday reprised the many inconsistencies in the prosecution case, including evidence that Lucio’s confession was forced and that Mariah died as a result of injuries sustained when she fell down a steep flight of stairs and not at her mother’s hands.

Leach has been at the forefront of efforts by Texas lawmakers to persuade the authorities to postpone the execution. He orchestrated a letter to the board of pardons signed by 80 House members, 32 of whom are Republican. A similar letter has been sent by 20 Texas senators, eight Republican.

In the House letter, the lawmakers pointed out that Lucio was treated by prosecutors in a completely different way from her husband, who was also responsible for Mariah’s care. Lucio had no previous history of violence and her children said she had never been abusive towards them; by contract her husband had a history of assault yet is now a free man having only served a four-year sentence for child endangerment.

The exceptional degree of bipartisan agreement, with more than half the legislature backing calls for a stay, is extremely rare in such a riven state. Leach, however, said he was unsurprised.

“I know our country is pretty divided, and we have our fights,” he said. “But when something is as clear as this, I know my legislative colleagues on both sides of the aisle are willing to do the right thing.”

Leach was one of several Texas lawmakers who visited Lucio earlier this month in the Mountain View Unit in Gatesville where the women’s death row is housed. He told the Guardian that they prayed together and he held her hand.

“Her faith is incredibly strong. She is comforted by the prayers of millions across the world, so it was an amazing moment.”

At the encounter, Lucio invited Leach to attend her execution on Wednesday should it go ahead. “If it does go forward, I will be there,” he said. “But I’m hoping that’s a trip I don’t have to make.”