The Supreme Court will be asked to allow terminally ill Canadians who face unbearable suffering the right to a physician-assisted death in a hearing Wednesday in Ottawa – a decision that, if granted, would overrule the wishes of Parliament and the court's own ruling from 20 years ago.

The case could usher in an epoch-making change on par with the Supreme Court's striking down of the criminal law on abortion in 1988, or the Ontario Court of Appeal's recognition of gay marriage in 2003.

The nine justices will hear from Lee Carter, of Vancouver, who took her 89-year-old mother, Kathleen Carter, to Switzerland for an assisted death in 2010. Her mother suffered from the painful degenerative condition of spinal stenosis, and said she did not wish to live life "as an ironing board," flat on her back, unable even to hold a newspaper.

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In an opening statement filed with the Supreme Court, Ms. Carter says the case is about "those who know there are states of being literally worse than death and wish to embrace the latter in the time and manner of their choosing." She is among several people who brought the court challenge, saying she shouldn't have had to face the fear of criminal prosecution for helping her mother die in the way that she wished.

Complicating the case, however, is that it pits one group of vulnerable people, the terminally ill who are in pain, against another vulnerable group, disabled people who fear they could be subject to pressure to accept suicide. It is largely that second group that the Canadian government says it speaks for, in trying to preserve life.

"An absolute prohibition sends the message that all lives are valued, and worthy of protection from those who may subtly encourage vulnerable people to terminate their lives," the government says in court documents.

The case highlights how the country's most powerful court now determines some of the basic decisions Canadians may make about their lives, or in this case, their deaths, under new powers given to judges by the 1982 Charter of Rights and Freedoms.

"The bottom line is, because you have those competing vulnerable interests, the court's decision will be no less political than a legislative decision," said Christopher Manfredi, a McGill University political scientist, and dean of the Faculty of Arts. "The court will look at society and come down with its own understanding of what constitutes a socially beneficial result."

The court is explicitly being asked to respond to what some say are changing social values around personal autonomy and death, even though it means rethinking its own decision of 1993, when it rejected a claim by 42-year-old Sue Rodriguez of British Columbia to a right to doctor-assisted suicide.

"This is one of those large issues, like abortion, like gay marriage, where either the criminal law or other elements of law have been used to suppress, to repress, various practices in society that were deemed to be immoral, socially undesirable, under prevailing sets of values," said Joel Bakan, who specializes in constitutional law at the University of British Columbia.

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Two decades ago, Ms. Rodriguez, who suffered from the degenerative nerve disease amyotrophic lateral sclerosis (ALS), became a household name when she challenged the ban on doctor-assisted suicide. The Supreme Court ruled 5-4 against her claim, saying that the state had a right to protect the sanctity of life by insisting on an absolute ban on assisted suicide. (She died a year later, helped by an anonymous doctor in Victoria.)

But much has changed since then. Several jurisdictions, such as Luxembourg and the Netherlands, and some states in the U.S., now allow doctor-assisted death. Justice Lynn Smith of the B.C. Supreme Court found after extensive hearings that those laws minimize the risks that vulnerable sick or old people will be pressured to accept suicide. She ruled that Kathleen Carter had a right to a doctor's assistance in ending her life. The province's Court of Appeal later overruled her, saying the courts could not go against precedent in the Rodriguez case.

Public opinion seems to be more accepting of assisted suicide, and that matters to the Supreme Court, legal observers say.

"The court doesn't want to lag too far behind and doesn't want to get too far in front," said Bernard Dickens, a professor emeritus of health law and policy at the University of Toronto.

The Supreme Court reserves the right to reject its own precedents, and has publicly acknowledged having done so several times in the past 20 years. Chief Justice Beverley McLachlin is the only judge remaining on the court from the Rodriguez case. She supported the right to assisted suicide.