The faltering, error-filled execution attempt in Oklahoma on Tuesday night came at the end of a day which began with a new study demonstrating that an unknowable number of people on death row are or were innocent. The combination could mean that the two-decade long trend in public opinion away from the death penalty will accelerate.

Almost no one — not even Oklahoma's governor, Mary Fallin — is defending what happened in the execution of Clayton Lockett, convicted of the 1999 murder of a Tulsa high school student. It followed a constitutional crisis between the governor and the state Supreme Court, which blocked the planned execution of Lockett and another man because of questions about the untested drug cocktail that would be used. (Fallin refused to accept the court's ruling.) When the cocktail was applied to Lockett, he didn't die immediately, expiring from a heart attack 43 minutes after the process began.

There's a scale of support for / opposition to the death penalty that goes something like this.

  1. It is never OK.
  2. It is OK as long as no innocent people are executed.
  3. It is OK as long as the process of being sentenced to death is fair.
  4. It is OK as long as the execution itself is conducted humanely.
  5. It is OK as long as the person's crime warrants such a harsh penalty.
  6. It is OK.

The botched Lockett execution is unsettling to people who lie on Point 4 of that scale. It's easy to see from the reactions to what happened to Lockett where people fall. Conservative commentator Erick Erickson, for example, tweeted, "If we could just go back to hangings or a firing squad, we wouldn’t have to wring our hands over how humanely we execute savage murderers." He's at Point 5.

The resolution of death sentences. (PNAS)

Earlier in the day, a new study gave ammunition to those at Point 2 (and, really, Point 1). It offered a statistical analysis of death row inmates between 1973 and 2004, finding that some 4 percent were likely innocent. The study looked, in part, at the number of times people on death row had been exonerated or switched to life sentences following evidence suggesting or demonstrating their innocence. Since 1973, more than 140 death row inmates have been exonerated. The reactions to this, too, are revealing; some argue that, without proof that an executed person was innocent, it's not fair to assume that any were.

Last October, Gallup released the latest in its long series of polls documenting American attitudes about the death penalty. The overview can be seen below. Since the mid-1990s, support for the death penalty has consistently fallen — a trend that, coincidentally or not, mirrors the decline in crime rates in the United States.

It may also be in part due to increased awareness around the disparities in the application of the death sentence, which speak to Point 3. There are disparities of race: "56% of death row inmates are black or Hispanic," as David Love wrote at The Guardian several years ago. "However, although racial minorities comprise half of all murder victims nationwide, a far greater proportion (77%) of the victims in capital convictions were white." There are disparities of geography: Pew Research recently created a map showing each execution since capital punishment was reinstated in 1977. The bulk have occurred in southern states, which is in part because several states, like Illinois, have enacted moratoriums on the practice. (Update: A reader reminds me that Illinois ultimately banned executions.) Nor was Lockett the first prisoner to suffer during his execution. In 2006, a prisoner in Florida experienced a perhaps more painful death, and there have been others.

As with other politically potent topics, the death penalty has become something of a litmus test. But for those considering how and when the death penalty is sentenced and applied, what happened on Tuesday is far more likely to move them up the scale presented above than down it. If the trend presented by Gallup continues, the death penalty will become less and less common in the United States. April 29, 2014, may end up being one a key day in accelerating that change.