PORTLAND — Dennis Dechaine’s bid for a new trial hinges on a fragment of DNA from a thumbnail clipping.

At a hearing next week, Dechaine’s court-appointed lawyer will try to convince a judge that the jury would have acquitted Dechaine of the 1988 kidnapping and murder of 12-year-old Sarah Cherry had it known about the DNA.

Dechaine has previously made four unsuccessful appeals, and his lawyer, Steve Peterson, has characterized this effort as Dechaine’s last, best chance.

Dechaine, 54, is serving a life sentence for the crime he says he did not commit. His defense contends that the unidentified male DNA extracted from the nail clipping from Cherry’s body points to the real killer. The state maintains that the thumbnail clipping was likely contaminated and that the DNA is irrelevant.

The scope of the hearing, scheduled to begin Tuesday before Superior Court Justice Carl Bradford, has narrowed since last year.

Bradford, the original trial judge, has denied Peterson’s motion that he recuse himself, as well as another motion to allow time-of-death evidence at the upcoming hearing. Bradford granted a request to run the partial DNA profile from the thumbnail against those in a state database of convicted felons, but that process produced no viable alternate suspects.

“What we’re left with is that the DNA in question does not match my client,” Peterson said.

The DNA comparison was made against a state database of 21,000 samples from convicted felons. The DNA profile from the thumbnail is incomplete and there isn’t enough genetic material to identify the donor conclusively, but it can rule people out.

About 80 possible matches were made, and some names were quickly eliminated because the people were too young or hadn’t been born at the time of the crime, Peterson said. The list was further whittled down by investigators for the state and the defense, he said.

Cherry’s family, lawyers who handled evidence, law enforcement officers on the case and state medical examiner’s staff members had already been ruled out as possible sources of the DNA.

Deputy Attorney General William Stokes said the state already knows who Sarah Cherry’s killer is, as did the jury in Dechaine’s trial.

“The evidence against Dennis Dechaine is literally overwhelming, and this evidence is meaningless,” he said.

Cherry was abducted from a baby-sitting job in Bowdoin on July 6, 1988. A search for her and Dechaine, then a 30-year-old farmer from Bowdoinham, began after a notebook and a truck repair bill with Dechaine’s name were found in the driveway of the home.

About five hours later, Dechaine was seen walking out of the woods about three miles away. Police found his pickup truck on a nearby discontinued logging road later that night.

Cherry’s body was found two days later, about 450 feet from where Dechaine’s truck had been. Her hands were bound in front of her, she was gagged and a scarf was wrapped around her mouth and neck. She had been sexually assaulted with birch sticks and stabbed in the temple, neck and chest.

Dechaine said he went into the woods to inject speed and was alone and lost. He maintains that someone took his papers, the rope that was used to tie the girl’s wrists and the scarf from his truck.

DNA profiling was new at the time of Dechaine’s trial in 1989. Bradford denied a pre-trial DNA testing request that year. Dechaine filed two unsuccessful appeals based on DNA, in 1995 and 2000.

The Maine Legislature passed a law in 2001 giving prisoners the right to seek new trials based on DNA evidence.

Lawmakers revised the law in 2006 to reduce the burden on the prisoner, from having to prove that the DNA belonged to the real perpetrator to showing that the evidence likely would have resulted in an acquittal.

Much of next week’s hearing will focus on the chain of custody of the thumbnail clipping, the potential for its contamination and the DNA testing. The hearing is expected to last through Wednesday and possibly into Thursday.

Among the possible witnesses is Thomas Connolly, Dechaine’s lawyer during the trial.

Connolly learned in 1992 that exhibits would be destroyed unless they were picked up. He had the nails tested for DNA at a lab in Boston before returning them to the state. The lab results, which came back in 1994, showed a mixture of Cherry’s blood and DNA from someone who was not Dechaine. It’s not known whether the unknown DNA came from blood, saliva, skin or another source.

Some testimony is likely to describe the conditions of autopsies at the time.

“They were not taking any precautions for DNA at the time. DNA was hardly thought of,” Stokes said. “Things were dramatically different.”

The prosecution notes that equipment, including nail clippers, was reused and subjected to contamination. The defense says the state relies on information from the labs in pursuing cold cases.

Dechaine has another case pending in Knox County Superior Court for a charge of trafficking in contraband at the Maine State Prison.

Dechaine is accused of illegally possessing morphine and the prescription anti-anxiety medication Klonopin. He told the Portland Press Herald that he attempted suicide in April 2010.

Jury selection is scheduled to begin Friday. The case is not expected to affect scheduling in the hearing for a new trial.