A Supreme Court ruling last week to once again block the execution of a Houston killer may finally force the Texas Legislature to set a clear standard for whom the state deems mentally unfit for the death penalty.

Bobby James Moore has been on death row in Texas for nearly 40 years. (TDCJ)

With its decision, the U.S. high court vacated a ruling from Texas’ highest criminal appeals court that would have allowed the execution of a man experts say is intellectually disabled.

Tuesday's ruling was the second time the court stopped the execution of Bobby James Moore, who in 1980 killed James McCarble, a 72-year-old grocery store clerk, during a Houston robbery.

Moore, 59, has been on death row for nearly four decades.

The Supreme Court prohibited executions of intellectually disabled people in 2002, calling them "cruel and unusual punishment," but allowed states to set their own standards for what constitutes mental disability.

Now, more than 15 years after its adoption, Texas' standard, called the Briseño rules, may finally be on its way out. Critics refer to those rules as the "Lennie standard," named for the simple-minded character Lennie Small in John Steinbeck's Of Mice and Men.

The nation's highest court in 2017 overturned a Texas appeals court's ruling that Moore was not intellectually disabled, saying its use of the Briseño standards was unacceptable. The state court delivered the same decision on Moore in 2018, this time saying it had not used Briseño factors.

The Supreme Court disagreed.

“Texas has had a long history of problematic verdicts including death penalty verdicts with defendants who were low-functioning,” said Harris County District Attorney Kim Ogg, who opposes the death sentence for Moore. “We were on the right side of history.”

In its 6-3 decision, the Supreme Court said Texas' judgment of intellectual disability relied too heavily on "lay stereotypes."

James Ellis, now a law professor at the University of New Mexico, argued the pivotal 2002 Atkins v. Virginia case in which the Supreme Court barred executing the mentally disabled.

It was Daryl Atkins' case that persuaded the Supreme Court to exclude mentally disabled killers from the death penalty. Atkins killed an enlisted airman for beer money in 1996. He has a reported IQ of 59. (AP file photo)

“Two years ago, the court said you can’t substitute clinical understanding with something you find more convenient,” Ellis said. “The decision two days ago said, 'No, we really meant it.'"

In defining “mental retardation” under Briseño, the appeals court named Lennie Small as someone most Texans would agree is mentally disabled and therefore should not be executed.

John Blume, a professor of trial techniques and director of the Cornell Death Penalty Project at Cornell University, said Lennie represents a stereotype of the intellectually disabled as simpletons.

Medical professionals agree that in reality, intellectually disabled individuals may be verbally competent, working individuals and socially capable.

“Texas, in a sense, stands alone because it was so clearly basing its judgment on nonscientific factors,” Ellis said.

The Supreme Court's latest decision on Moore does not offer a framework to replace the Briseño standard. That responsibility falls to elected representatives, and legislators have been slow to introduce a new standard in the nearly two decades since the Atkins ruling.

“The Briseño standard is just a court standard that they came up with in the absence of the Legislature doing anything," said Christi Dean, chief of the Capital Murder Division in the Dallas County public defender’s office.

Public defenders Brad Lollar (left) and Christi Dean served on the defense team of Erbie Lee Bowser (right), who was sentenced to life in prison for a rampage that left four women dead in 2013. (File photo)

Now, legal professionals are hopeful the Supreme Court’s latest decision on Moore will spur Texas legislators into action.

“The Legislature has so far declined to act,” said Brad Lollar, assistant chief of Dean’s division. “I’m hoping with this latest opinion the Legislature will take up the issue and give us some written laws that comport with the Atkins and Moore rulings."

In fact, a bill designed to do that is gaining momentum in the Texas House after last week's ruling on Moore's case. Two Republican lawmakers signed on to House Bill 1139 on Wednesday, the Texas Tribune reported.

The bill would allow pretrial hearings to determine the intellectual fitness of capital murder defendants, and determinations would be based on “prevailing medical standards.”

“The Court of Criminal Appeals has wrestled with this issue for the 30 years I’ve been a lawyer,” Ogg said. “It will be good to have some finality on the topic.”