Alabama failed to prove that its exclusion of chaplains from the execution chamber was necessary for the "compelling state interest" of security, wrote Kagan, joined by Breyer, Sotomayor and Barrett.
Previous practice by Alabama and other states "shows that a prison may ensure security without barring all clergy members from the execution chamber," Kagan wrote. "Until two years ago, Alabama required the presence of a prison chaplain at an inmate's side."
In another case in Texas, the Supreme Court recently ruled favorably toward Catholic death row inmate Ruben Gutierrez, who had challenged the state's prohibition of chaplains at executions. In the case, the court sent the matter back to the lower courts for reconsideration; a district court had found that the presence of a chaplain in the execution room would not be a security concern.
"So the State cannot now execute Smith without his pastor present, to ease what Smith calls the 'transition between the worlds of the living and the dead,'" the justices wrote on Thursday.
In 2019, the Alabama Department of Corrections banned clergy members from being present with people being executed by the state.
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The rule was put in place following a controversy in Texas in which members of some religions were allowed to have ministers at their executions, but others were not. Texas and Alabama then both implemented bans on all clergy during executions.
Furthermore, the federal government conducted more than 10 executions since last summer at which clergy were present, Kagan said.
"The law guarantees Smith the right to practice his faith free from unnecessary interference, including at the moment the State puts him to death," said Kagan.