LITTLE ROCK, Ark. (AP) -- The state of Arkansas is preparing to execute its first female inmate in more than 150 years, a former nurse who says she wants to die for suffocating her two children.

Christina Marie Riggs, 28, would be the fifth woman executed nationwide since the U.S. Supreme Court lifted a ban on capital punishment in 1976. She is to die by injection Tuesday night.Riggs asked the jury to sentence her to death for the Nov. 5, 1997, slayings of Justin, 5, and Shelby Alexis, 2. Justin was injected with potassium chloride and morphine and eventually smothered; his sister was smothered.

"I want to die. I want to be with my babies. I want you to give me the death penalty," she told jurors in 1998.

After her conviction, she pursued appeals at her family's request but withdrew them this year after a judge ruled that she could. The Arkansas Supreme Court later upheld her request.

Gov. Mike Huckabee could grant an unlimited number of 30-day reprieves, but cannot grant executive clemency to Riggs, because she has not asked for mercy, Huckabee spokesman Jim Harris said Monday.

Amnesty International USA and the American Civil Liberties Union have asked Huckabee to intervene to spare her life. The ACLU on Monday said Riggs was mentally incompetent, though courts have found her sane.

Riggs admitted killing her children and claimed she was deeply depressed at the time.

A woman has never been executed under state authority, the Arkansas Correction Department said, but Lavinia Burnett was hanged in 1845 for being an accessory to murder. Counties handled executions at that time.

Marlin Shipman, an Arkansas State University journalism professor who has been studying capital punishment of women, said society and government have often grappled with executing women.

"One of the things we saw was governors expressing a lot of soul-searching and concern about executing a woman," he said. "There was a pretty healthy debate as to whether women should have a special place."

He said the concern dwindled as the women's equality movement moved forward. Four other women have been executed nationwide since 1976, most recently Betty Lou Beets of Texas in February.

In Riggs' case, prosecutors told jurors that her children had become an inconvenience to her. They said she left the children by themselves while she competed in karaoke contests and plotted their deaths for two or three weeks.