Christy O'Donnell of Santa Clarita, Calif., is terminally ill with cancer. (Patrick T. Fallon/For The Washington Post)

After she was diagnosed with terminal cancer, Christy O’Donnell, 47, quickly became the new face of California’s right-to-die movement.

She was interviewed by Katie Couric and People magazine. She appeared on CNN and NPR and the front page of the San Francisco Chronicle. And this month, when Gov. Jerry Brown (D) signed a measure to legalize physician-assisted suicide, O’Donnell spent the day talking to reporters, sometimes from bed, always through excruciating pain.

But O’Donnell is unlikely to benefit from Brown’s decision. The law, which will vastly expand the share of the U.S. population with access to life-ending drugs, will not take effect until at least April. And with opponents threatening to try to repeal the law on a 2016 ballot, O’Donnell and others like her could be left in limbo for at least another year.

“I’ve always known that it was unlikely that I would be able to take advantage of the law in California in my lifetime,” O’Donnell said during an interview in her sunlit, beach-themed living room. But, she added, “very rarely in a human being’s life are they given the opportunity to make positive change in their country.”

Eventually, the law will permit doctors to prescribe life-ending drugs to terminally ill patients who meet certain requirements: California residency; repeated requests, both verbal and written, for the drugs; and a determination that the patient is mentally fit to make such a decision.

Its passage was a major victory for a movement that has sought for decades to gain traction in state legislatures. The California law quadruples the share of Americans living in states where physician-assisted suicide is permitted. The others are Oregon, Montana, Vermont and Washington.

It also creates a large and diverse testing ground for the practice.

“As California goes, so goes the nation,” said Jessica Grennan, national director of political affairs for Compassion & Choices, a nonprofit group that played a central role in California and is pushing similar measures in Colorado, Maine, Maryland, Massachusetts, New Jersey and New York.

Nationally, 68 percent of Americans support assisted suicide, a 10-point rise from 2014, Gallup reported in May. That increase is “consistent with changing attitudes related to a number of once-controversial social issues,” according to Gallup, which has also recorded a significant increase in support for same-sex marriage and marijuana legalization.

In California, the new law has 65 percent approval among statewide voters, according to a recent Field poll that found majority support for assisted suicide regardless of age, ethnicity or political party. The law has majority support among Catholics, the poll found, though the Catholic Church was vocal in its opposition.

A coterie of activists is nonetheless laying plans to gather signatures to try to repeal the law through a ballot measure next fall. Among them is Stephanie Packer, a 32-year-old mother of four who has her own terminal diagnosis.

Packer has scleroderma, a disease that causes tissue to harden or thicken. Scar tissue has formed in her lungs, so she breathes through a set of tubes connected to a small oxygen tank in her purse. She relies on a network of friends and family for everything from emotional support to groceries, she said from the living room of her childhood home in Orange, Calif., a few miles from where she lives today.

When someone is dying, “the world should rally around you and lift you up. That’s not when they give you this easy way out,” she said. If assisted suicide is an option, it will be “easier to give in to that all-consuming feeling when you find out that your life is closing.”

Packer also worries that introducing death as an option would pervert the conversation about treatment, a concern echoed by advocates for the disabled as well as the American Medical Association.

“Instead of participating in assisted suicide, physicians must aggressively respond to the needs of patients at the end of life,” the AMA says in its guiding ethical opinion on physician-assisted suicide. “Patients should not be abandoned once it is determined that cure is impossible.”

To make their case, both sides point to the experience in Oregon, where 859 people have died under the nation’s first “death with dignity” law, enacted in 1997. Virtually all of them were white, and nearly half had at least a college degree. Proponents say that shows initial fears about the law — that it would be used primarily by poor or minority patients without access to good health care — were unfounded.

Opponents counter that some patients may have been coerced into agreeing to take lethal medication. And they note that doctors are often wrong when they predict that someone will die within six months, the prognosis necessary to obtain a lethal prescription. Of 105 patients in Oregon who ingested lethal medication last year, 11 had obtained the prescription in 2012 or 2013.

Among patients who took the drugs, 92 percent cited loss of autonomy as an end-of-life concern. Other reasons for choosing assisted suicide: a diminished ability to participate in the activities that make life enjoyable (89 percent), loss of dignity (79 percent), loss of control of bodily functions (50 percent), being a burden on family, friends and caregivers (40 percent) and pain (25 percent).

Oregon played a supporting role in advancing the California law. Brittany Maynard, a young Californian with terminal brain cancer, move to Oregon last year because she could not end her life legally in her home state. Maynard’s story made international headlines and lent the right-to-die movement an attractive and sympathetic face. Her family continued her campaign in Sacramento after her assisted death in November 2014.

In May, the California Medical Association gave the movement another big boost by dropping its opposition to an “aid in dying” bill.

“Despite the remarkable medical breakthroughs we’ve made and the world-class hospice or palliative care we can provide, it isn’t always enough,” Luther F. Cobb, the association’s president, said in a statement at the time. “We believe it is up to the individual physician and their patient to decide voluntarily whether the End of Life Option Act is something in which they want to engage.”

After two decades of failed attempts to advance right-to-die measures, the California Senate passed the bill in June. Proponents then shelved the measure rather than risk losing a vote in the State Assembly.

“We were close,” said Sen. Lois Wolk (D), a sponsor of the bill. “We did not want a defeat, so we just stood down.”

A fresh opportunity emerged this summer when the assembly met in special session to address health-care funding. A temporary health committee was created, and its members were more supportive of the right to die. So advocates reintroduced the bill in August.

After an emotional debate, the law passed on Sept. 11. And Brown, a former seminarian, signed it, offering a surprisingly personal signing statement.

“I do not know what I would do if I were dying in prolonged and excruciating pain,” he wrote. “I am certain, however, that it would be a comfort to be able to consider the options afforded by this bill. And I wouldn’t deny that right to others.”

Bills passed in special session are governed by special rules, however: They can’t take effect until 91 days after the session ends. And the session will not formally end until at least January, when lawmakers reconvene. That means the law cannot take effect until April at the earliest.

Opponents of the law have until early January to collect enough signatures to force a referendum measure to repeal it. That would put the law in limbo until voters speak in November 2016.

O’Donnell and several other terminal patients are suing for the right to act sooner, citing their bleak prognoses. In the meantime, O’Donnell worries how the end will come.

She would prefer to go peacefully, in the bed where she and her daughter Bailey used to watch weekend cartoons. But her doctors say the end is likely to be grim: As O’Donnell’s liver fails, she will experience nausea, swelling and sharp abdominal pain. She is intolerant to morphine, the standard form of pain relief for people at the end of life.

For now, O’Donnell has found some serenity from a perch in her sunny living room, where she watches the hummingbirds and the shifting shade of the eucalyptus leaves. But the ability to leave life on her own terms, she said, would bring a deeper peace.

“You really have to weigh how much pain,” she said, “is worth four more weeks.”