WASHINGTON – A deeply divided Supreme Court ruled along ideological lines Monday that a Missouri inmate's rare medical condition isn't enough to stop his execution.

Nearly four years after upholding the use of lethal injections, the court's five conservatives ruled against Russell Bucklew's claim that it would cause him particularly gruesome pain, suggesting instead that it was just another delaying tactic.

"Mr. Bucklew committed his crimes more than two decades ago. He exhausted his appeal and separate state and federal habeas challenges more than a decade ago," Associate Justice Neil Gorsuch wrote. "Yet since then he has managed to secure delay through lawsuit after lawsuit."

The court's four liberal justices dissented. Associate Justice Stephen Breyer, its leading death penalty opponent, said executing Bucklew "risks subjecting him to constitutionally impermissible suffering."

"It may be that, as our nation comes to place ever greater importance upon ensuring that we accurately identify, through procedurally fair methods, those who may lawfully be put to death, there simply is no constitutional way to implement the death penalty," Breyer wrote for himself alone.

The high court's precedents from 2008 and 2015 upheld the use of lethal injection. In the latter case from Oklahoma, the court's conservative justices ruled the method had not been shown to be riskier than a known alternative.

Bucklew, 50, proposed the use of nitrogen gas, a method that has not been tested in any state, rather than those used in recent years: electrocution in Tennessee and firing squad in Utah. The state argued its one-drug lethal injection is safer.

Bucklew was convicted of murder, rape and kidnapping in 1996 and had not challenged his conviction or death sentence. Instead, he claimed that a rare and incurable condition that causes blood-filled tumors in his throat, neck and face creates the risk of extreme pain and suffocation.

In his majority opinion, Gorsuch ruled that Bucklew's proposed alternative wasn't feasible because it has not been tested anywhere and could not be shown to be any less painful than lethal injection.

"The Eighth Amendment does not guarantee a prisoner a painless death – something that, of course, isn’t guaranteed to many people, including most victims of capital crimes," Gorsuch said.

The high court blocked his execution twice before, first in 2014 following a series of botched lethal injections in other states, and again last March. The most recent action was by a 5-4 vote, with Associate Justice Anthony Kennedy siding with the court's four liberals.

Associate Justice Brett Kavanaugh succeeded Kennedy in October following a contentious confirmation battle that he won 50-48. During oral argument, both lawyers continuously pitched their arguments in his direction. But in the end, he ruled with the conservative majority.

Kavanaugh wrote separately to assert that the high court would have looked more kindly on Bucklew's request if he had suggested a plausible alternative method of execution – such as a firing squad.

"An inmate who contends that a particular method of execution is very likely to cause him severe pain should ordinarily be able to plead some alternative method of execution that would significantly reduce the risk of severe pain," Kavanaugh said. "At oral argument in this court, the state suggested that the firing squad would be such an available alternative."

In a footnote to his ruling, Gorsuch renewed the controversy over the court's decision in February to allow a Muslim inmate to be executed without the presence of his imam. Last week, the court delayed a Buddhist inmate's execution based on a similar religious request.

Domineque Ray "had long been on notice that there was a question whether his adviser would be allowed into the chamber or required to remain on the other side of the glass," Gorsuch wrote. "Yet although he had been on death row since 1999, and the state had set a date for his execution on November 6, 2018, he waited until January 23, 2019 – just 15 days before the execution – to ask for clarification."

The Supreme Court ruled 5-4 in 2015 that Oklahoma could use a more controversial, three-drug lethal injection protocol because challengers had not proven it would not mask excessive pain and had not identified a better alternative. That decision, written by Associate Justice Samuel Alito, prompted Breyer and Associate Justice Ruth Bader Ginsburg to suggest that capital punishment itself might be unconstitutional.

"While most humans wish to die a painless death, many do not have that good fortune," Alito wrote in 2015. "Holding that the Eighth Amendment demands the elimination of essentially all risk of pain would effectively outlaw the death penalty altogether."

Associate Justice Sonia Sotomayor wrote at the time that the ruling "leaves petitioners exposed to what may well be the chemical equivalent of being burned at the stake."