A condemned Missouri inmate faces a potentially "gruesome and painful" execution because of a rare medical condition that compromises the man's veins and causes multiple tumors in his head and throat, his attorney said Sunday.

Russell Bucklew is scheduled to die by injection Tuesday evening for killing a former girlfriend's new boyfriend in 1996 in eastern Missouri.

Bucklew, 49, was moments away from execution in May 2014 when the U.S. Supreme Court halted it amid concerns about Bucklew's medical condition. He suffers from cavernous hemangioma, a rare ailment that causes weakened and malformed blood vessels, as well as blood-filled tumors in his nose and throat.

Nearly four years later, the condition has worsened, attorney Cheryl Pilate said.

"Mr. Bucklew's rare and severe condition creates a very substantial risk of a gruesome execution, with choking and gagging on blood and the infliction of excruciating pain," Pilate said.

A panel of the 8th U.S. Circuit Court of Appeals has already refused to stop the execution. An appeal and stay request are pending before the U.S. Supreme Court. Pilate also has asked for clemency from Republican Gov. Eric Greitens.

Email messages seeking comment Sunday from spokespeople for Greitens and Missouri Attorney General Josh Hawley were not immediately returned.

The appeal offered a chilling possibility of what Bucklew's supporters believe could happen during the execution.

"Those highly sensitive tumors easily rupture and bleed," the appeal said. "As he struggles to breathe through the execution procedure, Bucklew's throat tumor will likely rupture," causing him to choke on his own blood.

"Bucklew's execution will very likely be gruesome and painful far beyond the pain inherent in the process of an ordinary lethal injection execution," the court document said, adding it would violate Bucklew's constitutional guarantee against cruel and unusual punishment.

In his response to the state Supreme Court, Hawley downplayed Bucklew's medical condition and cited continuing efforts over several years to put off the execution.

Bucklew's appeals also have suggested that if the execution is carried out, the state should use lethal gas instead of an injection of pentobarbital. Missouri law still lists gas as an option, but the state no longer has a gas chamber and has not used the method since 1965.

None of the 20 inmates executed since Missouri began using pentobarbital in 2013 have shown obvious signs of pain or suffering.

According to court documents, Bucklew was angry at his girlfriend for leaving him and moving in with Michael Sanders of Cape Girardeau when he tracked her down at Sanders' home in March 1996. He killed Sanders in front of the woman, her two daughters and Sanders' two sons and then attacked the woman and drove her to a secluded area and raped her.

After a state trooper spotted Bucklew's car, Bucklew shot at the trooper but missed, court records say. Bucklew later escaped from jail, hid in the home of the ex-girlfriend's mother and beat her with a hammer.

The execution would be the first in Missouri since January 2017.