By Judy Royal

WILMINGTON, North Carolina (Reuters) - Attorneys began making a case on Monday for why an Army doctor whose conviction for the murders of his pregnant wife and two daughters spawned several books and a television mini-series deserves a new trial 42 years after their deaths.

Jeffrey MacDonald, a former Green Beret whose crime prompted the bestseller "Fatal Vision," is serving three life sentences for the stabbing and clubbing deaths of his family members in their Fort Bragg, North Carolina, apartment in February 1970.

MacDonald, now 68, has long maintained his innocence and blamed a foursome of drug-crazed hippies for killing his wife, Colette, and daughters Kristen and Kimberley, ages 2 and 5. He described one of the intruders as a blonde woman in a floppy hat who he said chanted "acid is groovy; kill the pigs."

Courts have so far upheld his conviction, but MacDonald's lawyers are hoping for a different outcome as they present new evidence uncovered since his 1979 trial at a federal court hearing that could last up to 10 days in Wilmington.

Unlike in previous appeals, the 4th Circuit U.S. Court of Appeals ordered in April 2011 that Senior District Judge James C. Fox must weigh the new evidence in light of all the other evidence previously gathered rather than considering it piecemeal.

The evidence includes a sworn statement by a now deceased former federal marshal alleging that the lead prosecutor in the case intimidated a key witness into changing her testimony, and DNA test results of hair found by the bodies that did not belong to Jeffrey MacDonald or any of his family members.

On Monday, defense attorney Gordon Widenhouse said the claim by former Deputy U.S. Marshal Jim Britt and the unsourced hair samples would "compellingly demonstrate reasonable doubt." If jurors had heard those facts at MacDonald's trial, they never would have convicted him, Widenhouse said.

Prosecutor John Bruce reserved his opening statement for when the prosecution presents its evidence.

MacDonald, gray-haired and shuffling into the courtroom with chains around his ankles, waved to his now-wife Kathryn whom he married while in prison.

Much of the day's testimony came from Wade Smith, a Raleigh attorney who was part of MacDonald's defense team in 1979. Smith testified that Britt came to him out of the blue in 2005 to share information the former marshal said had weighed on him since the trial.

Smith said Britt told him that Helena Stoeckley, thought by MacDonald's backers to be the woman in the floppy hat, had admitted to him to being in the MacDonald home on the night of the murders and even described a broken hobby horse she had seen inside.

Smith testified Britt also said he heard prosecutor James Blackburn tell Stoeckley that if she told jurors she was in the family's house on the night of the murders he would charge her with first-degree murder. During the trial, Stoeckley testified that she could not remember what she was doing that night and had no recollection of being in the house.

Under questioning by Bruce, Smith said he never personally heard Stoeckley confess to anything that was useful to the defense or prosecution.

"She was being totally uncooperative in every way," Smith said. "She wasn't giving us anything."

Neither Stoeckley nor Britt is alive to testify at MacDonald's latest hearing, having died in 1983 and 2008, respectively.

Britt's wife at the time of the trial, Mary Britt, testified on Monday that he also told her about Stoeckley's admission to being inside the MacDonald home.

Mary Britt said she remembered her then-husband's response when she asked about the outcome of Stoeckley's testimony. "He said they can't use her testimony because her brain is fried from the use of drugs," she said.

(Additional reporting and writing by Colleen Jenkins; Editing by Vicki Allen and Lisa Shumaker)