South Carolina plans to execute a man convicted of two murders by firing squad, the first use of the method in the US in 15 years.
Brad Sigmon, 67, chose to be killed by a firing squad, saying he feared the alternatives of the electric chair or lethal injection would risk a slower and more torturous death.
Sigmon was convicted of beating to death his ex-girlfriend's parents, William and Gladys Larke, with a baseball bat at their home in the town of Taylors in 2001.
Executioners are due to strap him to a chair in a steel basin with a hood over his head and a target over his heart.
Three executioners planned to fire live ammunition from 4.5m away.
Sigmon asked the US Supreme Court on Wednesday to halt his execution, arguing that South Carolina's refusal to share information about its lethal injection protocol violated his due-process rights.
The last three men to be executed by South Carolina all chose lethal injection, and the executions lasted for about 20 minutes before they were declared dead, Sigmon's lawyer, Bo King, said in an interview.
Sigmon is scheduled to be executed at 6pm local time at the South Carolina Department of Corrections' execution chamber in Columbia.
There have been only three executions in the US by firing squad since 1976, all in Utah, one of only five states that still offers a method that was common in the 19th century during the Civil War.
Most US executions use lethal injection, introduced in the1970s as a less outwardly violent method.
But it has become the most frequently botched means of execution, according to the Death Penalty Information Center.
Some states have struggled to secure the drugs needed because of a European Union ban on pharmaceutical companies selling drugs for use in capital punishment.
Executioners have also sometimes struggled to find veins on prisoners' bodies and autopsies of people executed by lethal injection have sometimes found frothy, bloody liquid filling the lungs' airways, which some doctors say indicate the condemned person experienced the painful sensation of drowning before they died.
In January, the US Department of Justice cited those autopsy reports and withdrew its lethal injection protocol for federal executions, saying it may cause unconstitutional pain and suffering.
