Former Suffolk County sheriff Andrea Cabral endorses Andrea Campbell for mayor

Secretary of Public Safety Andrea Cabral.Pat Greenhouse/Globe Staff

Andrea Cabral was the Suffolk County sheriff when she got an urgent call one day from the twin sister of a desperately ill inmate.

The woman, whom Cabral didn’t know, explained that she hadn’t been able to see her brother, who was being held at the Lemuel Shattuck Hospital in Jamaica Plain. And she told the sheriff that Cabral’s department had to do a lot better by ill inmates and their families.

That call came from Andrea Campbell, then a young lawyer and now a city councilor running to unseat Mayor Marty Walsh. It made a deep impression.

“There was a steeliness and a determination, and I was impressed by the fact that she was speaking of herself, but she was speaking for other people,” Cabral said. “She was trying to right a wrong for someone she loved, but she was also trying to say something to me about the system itself.”

Cabral and Campbell became colleagues a few years later under Governor Deval Patrick, when Cabral was Secretary of Public Safety while Campbell was a member of the governor’s legal staff.

Now Cabral is endorsing Campbell for mayor — the most high-profile endorsement for either of Walsh’s opponents in the race next year.

“I think she’d make a fantastic mayor,” Cabral said. “She’s one of the best critical thinkers I’ve ever met in my life, and she uses that to build consensus.”

Campbell and fellow councilor Michelle Wu are opposing Walsh in his expected bid for a third term.

Cabral said her endorsement should not be viewed as a rebuke of Walsh, whom she described as a friend.

“I like Marty very much,” she said. “You make choices in elections. That’s the sum total of it.”

Cabral’s endorsement, particularly at this early stage of the race, is a boon for Campbell.

“When you’re challenging an incumbent — and I know this far too well — there are many folks who either want to manage relationships or don’t necessarily want to get involved too early because of the power of incumbency,” Campbell said. “So it really does require folks to exercise political courage — whether they’re endorsing me, hosting an event, holding fund-raisers — at this early stage in the game.”

But it’s also significant to win the backing of a longtime public safety official. Police reform is a key issue for Campbell and has been for her entire career. She’s currently pushing for greater diversity and transparency and more robust civilian review of the police.

She comes by this passion, in large part, because she has been affected by the criminal justice system her entire life, a fact that she has never hid. Her father was often on the wrong side of law, and a brother, Alvin Campbell Jr., is currently awaiting trial for a string of heinous sexual assaults allegedly committed while he was posing as a driver for Uber.

And then there was her twin brother, Andre.

As his sister moved through Boston Latin, Princeton, and UCLA Law School, Andre bounced in and out of jails and prisons.

He suffered from scleroderma, an autoimmune disease, and was receiving the indifferent medical attention our prison system often offers. But his sister became his fiercest advocate and protector, politely but firmly demanding to know why he wasn’t getting his medication, why he had lost 60 pounds, why she couldn’t see him.

He died at 29, in custody. But a fire to fight for justice for him and many others like him had been lit in his sister.

“Because of her background, her approach, and her life experience she (would bring) something to the role of mayor that no one who has held the office has ever had,” Cabral said.

Taking on an incumbent mayor is no small task. And next year’s race is at such an early stage that the mayor hasn’t even said publicly that he’s running.

Campbell is working to convince voters she can be a voice for a different point of view. And one influential observer has already bought in.