(CNN) -- The U.S. Supreme Court on Tuesday denied a stay of execution appeal from Teresa Lewis, scheduled to be the first woman executed in the United States in five years.
Virginia Gov. Bob McDonnell previously rejected a clemency request from the death row inmate, who is set to die by injection Thursday evening.
Lewis, a 41-year-old grandmother, pleaded guilty to her part in the 2002 slayings of her husband and stepson in their rural home near Danville, Virginia, about 145 miles from Richmond. Two male co-conspirators -- the triggermen -- were given life in prison without parole.
McDonnell last week refused to issue a stay for Lewis, who is the first woman scheduled to be executed in Virginia in nearly a century.
"Having carefully reviewed the petition for clemency, the judicial opinions in this case, and other relevant materials, I find no compelling reason to set aside the sentence that was imposed by the Circuit Court and affirmed by all reviewing courts," the governor wrote. "Accordingly, I decline to intervene and have notified the appropriate counsel and family of my decision."
Lewis and her lawyers had formally asked the governor to spare her life, arguing she has an IQ that is borderline mentally retarded and that she was manipulated to commit the crimes by a dominant male co-defendant. She pleaded guilty to her participation in the murders, but now regrets her actions.
"I just want the governor to know that I am so sorry, deeply from my heart," she recently told CNN. "And if I could take it back, I would, in a minute... I just wish I could take it back. And I'm sorry for all the people that I've hurt in the process."
Lewis had admitted to police she concocted the plot to kill her husband, Julian Lewis, and his son, C.J. Lewis, an Army reservist set to be deployed to Iraq. At the time, she was having an affair with Matthew Shallenberger and paid him and then-19-year-old Rodney Fuller to commit murder for the victims' insurance money. Court records show Lewis gave the two killers cash to buy weapons and left the door of the rural home open for easy entry.
Teresa was in bed with her husband when he was blasted with a shotgun by Shallenberger. Julian Lewis survived long enough to tell police: "My wife knows who did this to me." C.J. Lewis was killed by Fuller in his bedroom down the hall.
Despite her guilty plea, a state judge later sentenced Lewis to death while sparing the lives of triggermen Shallenberger and Fuller. The judge at sentencing called her "the head of this serpent."
The state argued Lewis was the mastermind of the murders-for-hire, and officials say she does not deserve mercy.
"I can frankly say that Teresa Lewis is as evil a person as I've ever met," said David Grimes, commonwealth's attorney for Pittsylvania County, who was at the scene of the crimes shortly after they occurred. "I would wager with some assurance that you wouldn't find anyone who knew her before this event occurred who thought she was mentally retarded, or had a limited mentality -- that it would ever cross their minds."
Her supporters say Lewis is deeply remorseful and has been a model prisoner, helping fellow female inmates cope with their circumstances.
"I do feel I could be a lot of help to some of the women -- which I have already. From my understanding I've already helped a lot, to change their lives, or made them look at their lives in a different way," she told CNN.
Amnesty International and best-selling author John Grisham are among those supporting leniency.
Grimes said Lewis has a "fairly low" IQ but noted courts have concluded she is not mentally retarded. The state also argued Lewis waited 45 minutes after the shootings before calling police and that she had involved her then-16-year-old daughter in the plot.
Defense lawyer Jim Rocap argued that such evidence suggested "Teresa could not have been the mastermind."
"Shallenberger has stated, and the experts that have examined her agree, that she was being used by Shallenberger, not the other way around," he said.
Lewis' attorneys say that Shallenberger admitted he used Lewis to get at the $250,000 she would receive in the event her stepson died. A letter from Shallenberger to another woman, they say, said that the only reason he slept with Lewis was "so she would give me the insurance money."
"She was exactly what I was looking for," he wrote. "Some ugly bitch who married her husband for the money and I knew I could get to fall head over heels for me."
Furthermore, they said, Shallenberger said he "manipulated the whole thing" and "knew he was going 'take' Lewis from the moment he met her," according to an affidavit from one of their investigators.
But Shallenberger, who committed suicide in 2006, refused to sign the affidavit and actually tore up and ate part of it.
Still, Grimes said, his investigation showed that Lewis took an active role in the plot, that she connived and manipulated everyone from her late husband to her lover to her children. From early on, he said, Lewis schemed several different ways to get the inheritance money. She helped plan an earlier plot to kill her husband that failed.
From CNN's Bill Mears