Lawyers for a Florida sex offender who was convicted earlier this year of raping and murdering 9-year-old Jessica Lunsford are set to appear before a judge Tuesday to argue that he should be spared a death sentence because he suffers from mental retardation.

A Miami jury convicted John Evander Couey in March of kidnapping, rape and murder for abducting Jessica from her Florida home in 2005 and keeping her in his room before burying her in garbage bags behind his trailer.

During the guilt phase of Couey's trial and his sentencing hearing, his lawyers argued that Couey, 48, suffered from mental retardation caused by a "perfect storm" of brain damage, lifelong substance abuse and an abusive childhood. (VIDEO)

Based on Florida law and a 2002 decision from the U.S. Supreme Court, offenders with mental retardation, as experts in the field refer to it, cannot be executed.

But defense lawyers were unable to convince the jury that the alleged condition mitigated Couey's culpability in Jessica's death, and the panel voted 10-2 in favor of a death sentence.

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The jurors' recommendation is an advisory sentence. The ultimate decision rests with 5th Circuit Judge Richard Howard, who will hear evidence from both sides Tuesday to determine whether Couey deserves mercy. Howard is expected to take the evidence and arguments under advisement and issue a formal ruling on Aug. 10.

If Couey is not sentenced to death, he will receive a sentence of life without parole.

In a separate part of the proceeding, Jessica's relatives will also have an opportunity to deliver victim-impact statements. Mark Lunsford, Jessica's father, is expected to make his first statement on the record about the loss of his daughter.

To earn a reprieve from a death sentence, lawyers for Couey, who spent most of his trial doodling in coloring books, must convince Howard that he actually suffers from mental retardation, a burden that experts in the field describe as an "uphill battle."

"It takes an open-minded jurist who won't mind the fact that people are going to say, 'How could you let that monster go?'" said Harry Simon, a federal public defender in Sacramento, Calif., who tried the state's first hearing on behalf of a death row inmate on the issue of mental retardation.

There are no official statistics on the success rate of Atkins claims, which takes its name from the Supreme Court case of Daryl Atkins v. Virginia, in which a Virginia death row inmate successfully argued that executing him for robbing and killing a man amounted to cruel and unusual punishment because of his mental retardation.

But Simon, who recently argued a landmark case in California involving death row inmate Anderson Hawthorne, estimates that less than 10 percent of the claims that have merit are actually successful.