(Reuters) - A decorated Vietnam War veteran convicted of murdering a Georgia sheriff's deputy in 1998 will be the first death row inmate executed in the United States this year if his sentence is carried out as planned on Tuesday night.

Andrew Brannan, 66, is scheduled to die by lethal injection at a state prison in Jackson, Georgia, at 7 p.m. EST unless a court decides to halt the execution.

Brannan's lawyers do not dispute that he shot Laurens County Deputy Kyle Dinkheller, 22, nine times during a traffic stop recorded by the deputy's patrol car video camera.

But they are seeking to have his life spared because, they argue, the severe physical and mental toll from his service in Vietnam as an Army forward artillery observer was not fully explained to a jury.

Brannan has suffered from combat-related post-traumatic stress disorder and was prone to flashbacks of the war, his attorneys said.

Georgia's Board of Pardons and Paroles on Monday refused to commute Brannan's sentence to life in prison without parole.

"The death of Deputy Sheriff Kyle Dinkheller was a terrible tragedy," said Joe Loveland, one of Brannan's lawyers, after the parole board's decision. "Executing a 66-year-old decorated Vietnam veteran with no prior criminal record who was seriously ill at the time of the crime only compounds the tragedy."

The Georgia Supreme Court denied a stay of execution on Tuesday afternoon, and Brannan's lawyers were expected to ask the U.S. Supreme Court to intervene.

By law, Georgia's governor does not have the power to grant last-minute clemency to a death row inmate.

Brannan received Army commendations and a Bronze Star, one of the highest individual military awards, for his service, according to his lawyers.

Kirk Dinkheller, the slain deputy's father, changed his Facebook profile picture on Tuesday to a photograph of his son's headstone.

"Nothing will ever bring my son back, but finally some justice for the one who took him from his children and his family," he wrote on the social media site earlier this month.

(Additional reporting and writing by Colleen Jenkins; Editing by Jeffrey Benkoe)