The following correction was printed in the Guardian's Corrections and clarifications column, Thursday 3 December 2009

The article below about attitudes towards the death penalty in Texas said the growth of DNA forensic evidence has seen nearly 140 death row convictions overturned across the US. This was based on a list of 139 cases compiled by the Death Penalty Information Center (DPIC) dating back to 1973. To clarify, the DPIC list notes that DNA played a substantial role in establishing innocence in 17 of the cases, not all of them as the sentence may have suggested

Even in Texas they are having their doubts. The state that executes more people than any other by far – it will account for half the prisoners sent to the death chamber in the US this year – is seeing its once rock-solid faith in capital punishment shaken by overturned convictions, judicial scandals and growing evidence that at least one innocent man has been executed.

The growth of DNA forensic evidence has seen nearly 140 death row convictions overturned across the US, prompting abolition and moratoriums in other states that Texas has so far resisted.

But the public mood is swinging in the conservative state, which often seems to have an Old Testament view of justice. A former governor, Mark White – previously a strong supporter of the death penalty – has joined those calling for a reconsideration of capital punishment because of the risk of executing an innocent person.

The number of death sentences passed by juries in Texas has fallen sharply in recent years, reflecting a retreat from capital punishment in many parts of America after DNA evidence led to the release of scores of condemned prisoners.

The number of death sentences passed annually in the US has dropped by about 60% in the past decade, to around 100.

"In Texas we have seen a constant stream of individual cases that really destroy public faith and integrity in our criminal justice system," said Steve Hall, former chief of staff to the Texas attorney general for eight years, who is now an anti-death penalty activist.

"You are seeing that scepticism reflected in a lot of different ways. You are seeing juries more reluctant to issue death sentences. You are also seeing a different approach by district attorneys. Some are breaking with the past culture of seeking the death penalty whenever they can."

A fortnight ago, two men sentenced to death and life in prison for the murder of four teenagers in 1991 were cleared after sophisticated forensic tests from the crime scene did not match either man.

Other prisoners are also being released after DNA evidence. In Dallas county alone, 24 people have been exonerated and the new district attorney has created a conviction integrity unit to examine other suspected miscarriages of justice.

Recent attention has focused on a high profile case which may become the first officially acknowledged miscarriage of justice which led to a man being executed.

The governor of Texas, Rick Perry, has been accused of gerrymandering a commission examining the evidence against Cameron Todd Willingham who was executed in 2004 for the murder of his three young daughters in an arson attack on his home. Perry abruptly replaced the chairman of the Texas Forensic Science Commission as it was about to hold hearings into a report by its own expert, who described the conviction as based on "junk science". The new chairman called off the hearing.

Other states have moved swiftly to address concerns about potential miscarriages of justice.

The release of four men in New Mexico prompted the governor, Bill Richardson, to abolish the death penalty in the state earlier this year, saying: "I do not have confidence in the criminal justice system as it currently operates to be the final arbiter when it comes to who lives and who dies for their crime."

Six years ago, the governor of Illinois declared a moratorium on the death penalty after realising that the state had freed more men from death row than it had executed since 1976.

Death penalty supporters in Texas claim the numerous appeal processes protect against a wrongful conviction. "No one who's involved in criminal prosecution has ever claimed they are absolutely perfect," said Dudley Sharp, founder of a Texas victims rights group, Justice For All. "But with the death penalty in the United States you have a system that protects innocence to a greater degree than a life sentence ever could."

But Hall says the highly politicised judicial system in Texas, with elected prosecutors and judges, is part of the problem. "One of the problems with having an elected judiciary is that you end up with judges who have to become good politicians. That means appealing to the voters. The presiding judge on the court of criminal appeals, Sharon Keller, ran as a pro-prosecution judge. That was her phrase," he said.

Keller - known has Sharon Killer to her critics because of her enthusiasm for the death penalty - is at the centre of a controversy that has further undermined confidence in the death penalty, after she refused to keep a court office open after 5pm to allow a last-minute appeal for a stay of execution while the supreme court decided on another case that affected all executions in Texas. The convict, Michael Richard, was executed hours later.

Keller is awaiting a verdict from the State Commission on Judicial Conduct on charges of dereliction of duty.

Earlier this year, Keller turned down an appeal from a man on the brink of execution, despite revelations that the judge and prosecutor at his trial had been having an affair.