(CBS/AP) COLUMBUS, Ohio - In an unusual plea for mercy ahead of his scheduled execution next month, a condemned Ohio killer acknowledged that he intended to sexually assault his girlfriend's 6-month-old daughter but said he never intended to kill the baby.

Attorneys for 46-year-old Steven Smith are planning to tell the Ohio Parole Board on Tuesday that the girl, Autumn Carter, died because Smith was too drunk to realize his sexual assault was killing the child. They said Ohio law is clear that a death sentence must involve an intent to kill the victim.

"The evidence suggests that Autumn's death was a horrible accident," his attorneys, Joseph Wilhelm and Tyson Fleming, said in a written argument prepared for the board. "Despite the shocking nature of this crime, Steve's death sentence should be commuted because genuine doubts exist whether he even committed a capital offense."

His attorney said Smith was never charged with sexual assault, meaning the jury's only choice was to convict or acquit him of aggravated murder.

The Richland County prosecutor said Smith continues to hide behind alcohol as an excuse, and calls Smith's actions "the purposeful murder of a helpless baby girl."

Prosecutor James Mayer told the board in his own written statement that the girl's injuries are consistent with a homicide that contradicts Smith's claim he didn't intend to kill the girl.

"The horrific attack upon Autumn Carter showed much more than Smith's stated purpose," Mayer said.

Mayer said Monday he didn't know why Smith wasn't charged with rape, but said it wasn't part of a trial strategy.

The attack happened early in the morning of Sept. 29, 1998, in the Mansfield apartment of the girl's mother, Kaysha Frye, a woman Smith was dating for about six months.

Frye was awakened after 3 a.m. by a naked Smith, who placed Autumn beside her in bed, according to records prepared for the parole board hearing. Frye realized the girl wasn't breathing, told Smith he'd killed her, and then ran to a neighbor's house for help.

Smith, known to consume as many as 12 beers a day, had several beers earlier in the evening and had a blood-alcohol content of .123 -- well above the legal limit -- when he was tested almost eight hours later at 11 a.m., records show.

Smith had unsuccessfully tried to have sex with his girlfriend the evening before the attack, according to records. The prosecutor argued that Smith's assault of the girl was revenge for Smith's failure to perform with Frye.

Smith's attorneys dispute this, saying the girlfriend was not upset with Smith.

Prosecutors presented evidence at trial that Smith's attack lasted as long as 30 minutes, during which time Smith beat the girl to death.

Expert witnesses for Smith conclude he may have accidentally suffocated the girl within three to five minutes while he lay on top of her, according to Smith's clemency petition.

Smith's attorneys have an uphill battle in their argument because of the "moral repugnancy" surrounding the claim of partial innocence, said Doug Berman, an Ohio State University law professor and death penalty expert.

"But if the lawyers for this defendant can legitimately assert that the evidence doesn't show or support that this was an intentional killing, not only is it appropriate to bring this up at clemency, I think they're obliged, representing their client appropriately, to stress this point," Berman said.