By Lawrence Hurley
WASHINGTON (Reuters) - Tensions on the Supreme Court over America's use of the death penalty boiled over on Wednesday as the justices appeared badly split in a case challenging Oklahoma's lethal injection method as a breach of the Constitution's ban on cruel and unusual punishment.
The nine-member court's five conservatives seemed likely to side with Oklahoma in the case brought by three death row inmates, while its four liberals expressed doubt about the propriety of using the drug at the center of the dispute.
Conservative Justice Anthony Kennedy, who often casts deciding votes in close cases, said nothing to suggest he would side with the liberals.
The testy nature of exchanges between the justices during the hour-long oral argument illustrated that while the case concerned just one drug, it was playing out against the much bigger question of whether the death penalty should be used in the United States at a time when most developed countries have abandoned it. But that question was not before the court.
The drug at issue is a sedative called midazolam, which the three convicted murderers - Richard Glossip, John Grant and Benjamin Cole - contend is unsuitable for use in executions because it cannot achieve the level of unconsciousness required for surgery.
They say using midazolam violates the Constitution's Eighth Amendment prohibition on cruel and unusual punishment.
Some of the conservatives expressed concern that the case is part of a sneak attack on the death penalty itself.
"Is it appropriate for the judiciary to countenance what amounts to a guerrilla war against the death penalty, which consists of efforts to make it impossible for the states to obtain drugs that could be used to carry out capital punishment with little, if any, pain?" Justice Samuel Alito asked.
The three-drug process used by Oklahoma prison officials has been under scrutiny since a botched execution exactly a year ago to the day of convicted murderer Clayton Lockett. He could be seen twisting on the gurney after death chamber staff failed to place the intravenous line properly.
Chief Justice John Roberts and Justice Antonin Scalia questioned whether Lockett's case was relevant because there was evidence the drug was not properly administered.
"I mean, if in fact the execution was not properly conducted, I don't see how you can blame it on the drug," Scalia said.
Despite the bigger question of the death penalty lurking in the background, only liberal Justice Stephen Breyer specifically referenced it.
Breyer said that "if there is no method of executing a person that does not cause unacceptable pain, that - in addition to other things - might show that the death penalty is not consistent with the Eighth Amendment."
But Breyer and the other liberals focused most of their questions on technical details about whether the drug should be used in the death chamber because it cannot maintain a coma-like unconsciousness, potentially leaving inmates in intense pain from lethal injection drugs that halt breathing and stop the heart.
Justice Elena Kagan said if midazolam does not work, condemned inmates would essentially be "burned alive" by one of the other drugs used in the process, potassium chloride.
Midazolam has been used in executions in Oklahoma, Florida, Ohio and Arizona.
Glossip arranged for his employer to be fatally beaten. Grant stabbed a correctional worker to death. Cole killed his 9-month-old daughter.
Oklahoma's governor this month signed a law allowing the use of nitrogen gas as an alternative execution method if the Supreme Court finds the state's lethal injection process unconstitutional or drugs are unavailable.
A ruling is due by the end of June.
(Editing by Will Dunham)