When Vice President Bush picked Massachusetts' "prison furlough" program as a rhetorical hammer to bash Gov. Michael S. Dukakis, Bush's likely opponent in the fall presidential campaign, he seemed to have his hands on a near-perfect campaign issue. The idea of giving "weekend passes" to imprisoned murderers sounded straightforward enough to explain in a single sentence -- just right for TV news -- and it seemed to place Dukakis well to the left of the mainstream.

In fact, the prison furlough question, which Bush has brought up repeatedly this week, is one of the most complicated aspects of criminology, with enough conditions and qualifiers to keep both presidential candidates engaged for months if they choose to continue the debate. And, according to academic and governmental students of prison policy, Dukakis' support for the concept of furloughs puts him squarely in the mainstream today among corrections officials from coast to coast.

Dukakis' state, Massachusetts, is one of more than 40, including Maryland and Virginia as well as Washington, D.C., that have furlough programs that allow prisoners -- including some convicted murderers -- to leave the walls for a day or more to go home or to work at an outside job.

California gave furloughs to convicted felons throughout the eight years Ronald Reagan was its governor; Reagan retained the policy even after two convicts committed murder while on furlough in 1972. The U.S. Bureau of Prisons says it given furloughs to 2,500 felons so far this year (of whom six did not return).

Bush apparently never criticized these programs until last weekend, when he cited the Massachusetts' furlough program as evidence that Dukakis and other Democrats "can't find it in their hearts to get tough on criminals."

When he was challenged on the point a few days later, Bush noted correctly that the furlough law in effect until this year in Massachusetts was more liberal than similar policies in most of the states. But the Massachusetts program Bush criticized was signed into law by a Republican governor, Francis Sargent. On the other hand, Dukakis supported that law strenuously, and resisted signing new legislation tightening the Massachusetts system until after he became a presidential candidate.

The complexities of the issue, though, may get lost as Bush blasts Dukakis on the stump and reminds voters of a convicted murderer who left a Massachusetts prison on furlough in 1987 and then committed a brutal rape in Maryland.

"Yes, it's buzzword politics," said Michael Robinson, a Georgetown University scholar who studies politics and media. "But it's a good issue for George Bush. Dukakis can give all the background and context he wants, but my guess is all those subleties will be lost when the audience hears that this man gave a murderer a free pass out of jail."

Bush's attack on furloughs dismays scholars who specialize in corrections policy. "Talking about murderers out on the street sensationalizes the facts," complained Robert Smith, a behavioral sciences professor at the West Virginia College of Graduate Studies and an expert on furlough programs. "I know it's politics, but it's so inappropriate."

The emergence of prison furloughs as a national campaign issue has sparked unprecedented activity at criminology research centers. Bureaucrats at the Justice Department's National Institute of Corrections said they've received countless calls seeking details on the national furlough situation. One of the calls was from Bush's research staff; that one was made Monday morning, two days after Bush first brought up the furlough issue in a campaign speech.

Formal programs permitting incarcerated convicts to leave prison spread across the country in the 1970s, when corrections officials began increasing their emphasis on rehabilitation as the proper goal of a prison system. By 1980, said Smith, the West Virginia behavioral scientist, "44 states, the federal system, and D.C. all had furlough policies in effect." Since then, he added, some states have tightened their furlough rules as part of a general effort to get tough on crime.

Corrections officials list several reasons for permitting prisoners to leave the walls periodically. Conjugal visits can help reduce tension, sexual and otherwise, in the inmate community, they said. And furloughs let inmates keep ties with the outside community and look for work.

Some states prohibit furloughs for prisoners who have committed crimes of "extreme violence" or those facing life imprisonment. But more than 33 states do permit "lifers" to leave prison on furlough, according to a survey taken last year by Marjorie Marlette, editor of a newsletter for prison officials.

The Massachusetts law that Bush attacked went further than other states in that it allowed furloughs even for prisoners who had been sentenced to life without the possibility of parole.

One of the prisoners who received furloughs under that Massachusetts law was convicted murderer Willie Horton, Jr. Horton was given 10 furloughs while imprisoned. Nine times he returned. On the tenth, he fled to Maryland and committed assault and rape. He is now serving a life term in Maryland; the sentencing judge there refused to send Horton back to Massachusetts for fear he might be furloughed again.

The Horton case sparked an uproar in Massachusetts. The controversy heightened when the Dukakis administration, citing confidentiality laws, refused to explain why Horton had been furloughed. Eventually, the state did release Horton's records. They showed he had drug and disciplinary violations in prison in the 1970s, but had good reports in the years before he was first furloughed in 1985.

The state legislature responded by tightening the state's furlough program. Dukakis said he personally opposed the new restrictions, but he signed the law this spring.

Dukakis has responded to the Republican charge that he is "soft on crime" by noting that he has instituted new anticrime programs. He said crime rates have fallen faster over the past five years in Massachusetts than in almost any other state. Federal Bureau of Investigation statistics bear out this assertion.

The figures show that reported crimes dropped markedly from 1982 to 1986 in Massachusetts and other jurisdictions, including the District of Columbia, that have experienced an economic boom. It is a common pattern for crime rates to go down in prosperous times, law enforcement specialists said.