(CNN) The state of Georgia is set to carry out its second execution of the year on Tuesday, when it plans to put to death Keith Tharpe, who was sentenced in 1991 for murdering his sister-in-law.

But Tharpe, 59, and his attorneys are seeking a stay of execution, based in part on racist comments a juror made after the trial had ended. Tharpe is black and the now-deceased juror who made the comments was white.

The attorneys are not claiming that Tharpe is innocent of the crimes for which he's been convicted. Rather, they are arguing that his death sentence should be overturned because of juror misconduct.

They say Tharpe's death sentence was the result of a racially biased juror who, in a post-trial interview seven years after Tharpe's conviction and sentencing, used the n-word and wondered "if black people even have souls."

Keith Tharpe shown in an undated booking photo from the Georgia Department of Corrections.

A biased juror, they argue, violates Tharpe's constitutional rights to a fair trial, guaranteed by the Sixth Amendment.

They also argue that the juror lied during jury selection, concealing the fact that he knew the victim's family. Furthermore, the attorneys say Tharpe is intellectually disabled, which would make it illegal for him to be executed under federal law.

The Georgia attorney general's office did not immediately respond to CNN's requests for comment.

The 11th Circuit denied Tharpe's appeal for a stay of execution on Thursday. His attorneys have filed a case with the Supreme Court of the United States.

After reviewing his case file, the Georgia Board of Pardons and Paroles on Monday denied his request for clemency, paving the way for his Tuesday execution.

Tharpe convicted of shooting sister-in-law

At the time of his crime, September 25, 1990, Tharpe and his wife were estranged. Prosecutors said Tharpe stopped his wife and sister-in-law in the road as they drove to work, according to court filings from the federal district court.

The documents say he took his sister-in-law, Jacquelin Freeman, to the back of the vehicle and shot her with a shotgun before throwing her into a ditch and shooting her again, killing her. An autopsy showed Freeman had been shot three times.

Prosecutors alleged Tharpe then raped his wife and took her to withdraw money from a credit union, where she was able to call police for help, according to the documents.

Three months later, convicted of malice murder and kidnapping, Tharpe was sentenced to death.

Juror expressed racist views

Tharpe's current case centers on the post-conviction testimony of Barney Gattie, a white juror in Tharpe's trial.

In May 1998, lawyers from the Georgia Resource Center, a nonprofit organization that offers free legal representation to prisoners on death row, conducted interviews with each juror as part of a routine investigation to prepare for Tharpe's petition for habeas corpus, the process of determining whether his imprisonment was unlawful.

Brian Kammer, Tharpe's attorney with the Georgia Resource Center, said Gattie showed in his interview that he "harbored very atrocious, racist views about black people."

Tharpe's lawyers claim Gattie, who is now deceased, used the n-word with the lawyers throughout the interview, in reference to Tharpe and other black people.

According to his affidavit, Gattie said, "In my experience I have observed that there are two types of black people: 1. Black folks and 2. "N****rs."

Gattie later said in a deposition that he did not intend to use the n-word as a racial slur, according to court documents.

Gattie went on to say in his affidavit, "I felt Tharpe, who wasn't in the 'good' black folks category in my book, should get the electric chair for what he did." As of 2001, Georgia carries out its executions by lethal injection.

"After studying the Bible, I have wondered if black people even have souls," Gattie said.

Weeks after the interview, Tharpe's attorneys returned to Gattie's home and read his statements back to him, periodically stopping to ask him if the statements were accurate, court documents say.

Gattie had only one correction, but the rest of his statement stood, court documents filed by Tharpe's attorneys say. He signed the 1998 affidavit under oath.

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"He basically admitted his criteria for deciding to sentence Mr. Tharpe to death had much more to do with his race than any of the facts of the crime," Kammer said.

But two days later, the state -- doing "damage control," Kammer said -- came back to Gattie and had him sign a second affidavit that undercut his statements to Tharpe's attorneys, claiming Gattie was drunk at the time he made them.

But, Kammer argues, "he never denied that he in fact held the views that he stated in the initial testimony."

During jury selection for the initial 1991 trial, Gattie told attorneys from both sides that he could be fair and impartial during the trial, Kammer said.

Recent SCOTUS rulings could help Tharpe

Georgia law states that juror testimony cannot be used to impeach the verdict, or render it invalid -- even if it involves racial bias, Kammer said. At the time Gattie made the statements in question, this rule kept Tharpe's attorneys from being able to use them to prove his death sentence was the result of racial bias.

In Georgia, defendants can only receive a death sentence if the jury reaches the decision unanimously.

But Kammer and his team are relying on some recent United States Supreme Court decisions to back their motion for a stay of execution.

The central one, Kammer told CNN, is Pena-Rodriguez v Colorado. In March, the US Supreme Court held in a 5-3 vote that laws like Georgia's are invalidated when a juror "makes a clear statement that indicates he or she relied on racial stereotypes or animus to convict a criminal defendant," Justice Anthony Kennedy wrote in the majority opinion.

Essentially, a juror's racial bias constitutes a violation of a defendant's rights to an impartial jury guaranteed by the Sixth Amendment, and prevents defendants from being able to prove a violation of their constitutional rights.

"A constitutional rule that racial bias in the justice system must be addressed -- including, in some instances, after the verdict has been entered -- is necessary to prevent a systemic loss of confidence in jury verdicts, a confidence that is a central premise of the Sixth Amendment trial right," Kennedy said.

Tharpe's request for a stay was denied by the 11th Circuit Court on September 21. A federal district court denied Tharpe's motion seeking a reopening to federal habeas proceedings on September 5, the day before the state issued a warrant for his execution.

Other aspects of attorneys' challenge

Gattie also said during jury selection he had no connection to Jacquelin Freeman's family. Tharpe's attorneys later discovered that wasn't the case.

In the same affidavit in which he made the remarks in question, Gattie also admitted to knowing the Freemans.

Both of Gattie's false statements during jury selection -- that he could be fair, and that he had no connection to the case -- constitute violations of Tharpe's right to an impartial trial under the Sixth Amendment, Tharpe's attorneys claim.

They are also arguing Tharpe is intellectually disabled, Kammer said, which would make it impossible for him to be executed under federal law. Tharpe has an IQ of "approximately 70," Kammer told CNN.

As Tharpe and his team wait for a decision by either the Supreme Court or the parole board, arrangements for his execution proceed as planned.