More than two-thirds of the executions involved convicts "who exhibited symptoms of severe mental illness, intellectual disability, or the effects of trauma or some combination of those," he said.
For instance, Georgia executed Andrew Brannan in January for killing a sheriff's deputy 17 years ago. Brannan was a decorated Vietnam War veteran whom the Department of Veterans Affairs considered 100 percent disabled because of posttraumatic stress disorder.
Also, some states resorted to illegal or questionable means of execution because they were unable to procure the normal drugs for lethal injection. Many pharmaceutical companies have stopped providing drugs for executions, and the European Union, which strongly opposes the death penalty, has banned the export of drugs for capital punishment.
Nebraska, Arizona, and Texas tried to import drugs for execution that were not approved by the Food and Drug Administration. The drugs were seized, and in one case Federal Express refused to transport them to Nebraska.
Some states then approved other means of execution – Utah brought back the firing squad and Oklahoma approved the use of nitrogen gas as a back-up method. Oklahoma saw a botched execution in January, and an autopsy later revealed that the wrong lethal drug had been administered.
All this undermined confidence in the states' authority to execute criminals, Dunham said. "The level of incompetence that was involved in that administratively botched execution was astounding," he said of the January execution of Charles Warner in Oklahoma. "Can you trust the states to carry this out in a fair, humane, and competent way?"
Dunham noted that while there are multiple factors behind the drop in support, the shift in moral sensibilities cannot be overlooked, along with the influence of the papacy.
Pope Francis made a "very strong statement against the death penalty in a very public setting," he said of the Pope's Sept. 24 address to a joint meeting of Congress, in which he called for a "global abolition of the death penalty" and offered "encouragement to all those who are convinced that a just and necessary punishment must never exclude the dimension of hope and the goal of rehabilitation."
Francis talked in a "graceful and humane way" that "appealed to our better nature," Dunham added.
The drop in public support "is not something new," explained Monsignor Stuart Swetland, president of Donnelly College in Kansas, saying that recent Popes have led the way in calling for its abolition. He cited St. John Paul II's homily in St. Louis in 1999, in which he urged Catholics to be "unconditionally pro-life" and called for an end to the death penalty.
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"A sign of hope is the increasing recognition that the dignity of human life must never be taken away, even in the case of someone who has done great evil. Modern society has the means of protecting itself, without definitively denying criminals the chance to reform," St. John Paul II said.
And in 2011, Benedict XVI expressed his hope for further abolition of capital punishment in countries worldwide.
Monsignor Swetland believes that these recent papal statements represent a development in the Church's teaching on the state's legitimate use of capital punishment.
"My theological opinion, as someone who teaches moral theology and social ethics in particular, is that we're undergoing a development of doctrine here," he told CNA.
When the Church's teaching has developed over time on issues such as the morality of slavery and torture, "we go from the more permissive to the less permissive," he said, "meaning that we come to recognize that the demands of charity and mercy and justice are more demanding than we thought before."
"And so while for a while Catholic teaching permitted slavery under some specific, restricted conditions, it came to see through faith and time – same thing with torture – through faith and time that it was always and everywhere wrong."