Prime Minister Stephen Harper is expected to announce changes on Wednesday to Canada's life penalty for convicted killers that would make life truly mean life behind bars – or as close as possible to pass constitutional muster.

Parents whose children were murdered in the past three decades have been invited to the announcement in Toronto.

Changes to the penalty for murder have been vigorously debated in the Justice and Public Safety departments and the federal cabinet over the past several weeks. Those changes are especially fraught, observers say, because how murder is punished sets the tone for the Canadian justice system. The government is struggling to keep its 2013 Throne Speech promise to hold some convicted killers in prison for life, while also respecting the Constitution's guarantee against cruel and unusual punishment.

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A stiffened murder penalty could make a potent symbol of the Conservative government's tough-on-crime approach heading into an election expected in October.

One proposal was life without parole for certain categories of first-degree murder, such as especially vicious planned killings. That was taken off the table, a source with knowledge of the process says, after government lawyers raised concerns that removing all hope of freedom would be rejected by the courts.

A second proposal would have extended the current waiting period of 25 years for full parole by at least a decade, and required a second layer of approval, this one from the justice minister – reverting to a political system of approval abolished more than a half-century ago. But that proposal, too, encountered resistance.

A third option, in which a jury could recommend to a judge that parole eligibility be set at anywhere from 25 to 40 years for some categories of first-degree murder, is now being studied by a House of Commons committee. It is contained in a bill proposed last year by Conservative MP Colin Mayes.

Under the current law, first-degree murder is automatically punishable by a "life" sentence. But first-degree killers are eligible for day parole after 22 years, and full parole after 25. If released, they need to report to authorities for life, and can be returned to prison if they violate their release conditions. First-degree murder includes planned and deliberate killings, killings of police officers and prison guards and killings that occur during a sex assault or kidnapping.

"I think society has a right to identify those that they think have committed such a horrendous act that we need to lock them up and throw the key away," John Muise, a former parole board member and former Toronto police officer, said in an interview.

If approved by Parliament, this would be the third major change in four years to the penalty for murder. In 2011, the government allowed judges to add together the parole waiting periods for multiple murders. Also in 2011, the government removed the "faint-hope clause" that allowed those convicted murderers to apply to a jury after 15 years for permission to seek early parole.

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Archie Kaiser, a professor specializing in criminal law at Dalhousie University's Schulich law school in Halifax, said any increase in the waiting period beyond 25 years would be disturbing and, in light of homicide rates that have fallen to their lowest point since 1966, difficult to justify.

"The proposed changes seem to be a function of crude instrumental politics, an effort to appeal to base impulses of vengeance. It doesn't seem very Canadian," he said in an e-mail.

New Democrat MP Françoise Boivin said the government's point is to show "'we're tough, we're against criminals." She said the most dangerous killers are already denied parole and held for life.

Liberal MP Sean Casey said the government must protect the public from Canada's most dangerous criminals. "If the government plans to table changes to life sentences," he said, "we will await the details and thoroughly review them as a caucus."