CANADIAN Minister of Justice A. Kim Campbell last week asked the Supreme Court to decide if a miscarriage of justice has occurred in the case of David Milgaard, one of the country's longest-serving prisoners.Mr. Milgaard has been imprisoned in Canada since his 1970 conviction for murder, but he and his supporters have long insisted that he is innocent. They have sought official review of his case and publicized information suggesting that another man may have committed the crime for which Milgaard was convicted. "It's a pretty good day," said David Asper, Milgaard's lawyer, just before a celebratory party at his firm's offices on Friday. He said a start-to-finish review of a criminal case by the Supreme Court, encompassing both new and old evidence, was virtually unprecedented in Canadian legal history. Supreme Court justices conducted a narrower review of a controversial criminal case almost 25 years ago, concluding that Steven Truscott, who had been convicted of committing a murder when he was 14 years old, was indeed guilty. Milgaard was convicted of killing a 20-year-old nursing assistant in Saskatoon, Saskatchewan. His legal appeals were exhausted by the end of 1971, but since then a key witness has recanted, physical evidence has been discredited, and Milgaard's supporters have uncovered evidence that points to another possible suspect. Ms. Campbell's announcement reverses a decision she made last February, when she said the case against Milgaard was solid and that there was "no basis to conclude that a miscarriage of justice may have occurred here." In a telephone interview Friday Assistant Deputy Attorney General Bruce MacFarlane, who has handled the Milgaard case for the minister, said Campbell was not changing her mind about the case. "The court will provide advice to the minister," he said, "and she will decide what the final decision is." But he added: "It's safe to say that a decision from the Supreme Court would be very persuasive" to Campbell. "We expect that [the case] will be assessed by the court quite expeditiously," perhaps next spring, in an "extensive public review" that would likely last at least two or three weeks, Mr. MacFarlane said. Speaking from his father's home in Winnipeg during an eight-hour prison furlough on Saturday, Milgaard said, "I feel happy [that] the decision has been finally got around to." But he said "the decision that came down didn't have the most important part: an opportunity for my release on bail." Nonetheless, the minister's decision sent the family's hopes "soaring," said Milgaard's mother, Joyce. Mr. Asper said he would begin work this week to have Milgaard freed pending the Supreme Court's review. MacFarlane said that Milgaard is only eligible for release on parole, but added that the prisoner's two short-lived escapes would continue to make him an unlikely parole candidate. Last February, Campbell refused Milgaard's request to reopen the case in light of new information, which Milgaard's lawyers filed in late 1988. Asper and his partner, Hersh Wolch, returned to the minister later this year with yet more information, much of it developed by Centurion Ministries, a non-profit Princeton, N.J., concern that investigates the cases of those who may have been wrongfully convicted. The case has also been scrutinized by a number of news organizations, including the Monitor, during the last six months, and has caught the attention of Canadian politicians. Prime Minister Brian Mulroney took a personal interest in the case last September, stopping to talk to Joyce Milgaard as she stood ready to picket one of his appearances in Winnipeg. Shortly after that, Milgaard was moved to a prison farm outside the castle-like institution where he has spent most of the past 22 years. Asper also says that a Nov. 11 meeting with Justice Department officials to discuss the case proceeded with unusual efficiency. "They were all ears," he said. Campbell has denied that her deliberations have been influenced by the growing public and political interest in the case.