For a time after a brain tumor sidelined her plans to run against Mayor Rahm Emanuel, Karen Lewis regretted missing her chance.

Lewis had long been determined to oust the mayor who had pushed her members to a launch a historic strike, cut schools’ budgets and once cursed her out over dinner. After searching in vain for a candidate, she was taking steps to take him out herself.

But her own body betrayed that wish, she said Wednesday, the first time she has publicly discussed her health and the treatment she’s undergoing for it.

“There was a moment where I was like really angry that I couldn’t do it,” Lewis said at the Chicago Teachers Union headquarters.

“Yes, I wanted to be in this race, I’m not gonna. . . . But at this moment in time, no, I don’t miss not being in it at all,” she said. “What happened was, it was clear to me it wasn’t my time, this wasn’t for me.

“I tell people all the time, there’s an old Jewish joke, ‘If you want to make God laugh, tell him your plans.’

“So I felt like I made God laugh, and he said, ‘Sit down before you break something.’ ”

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Lewis, 61, had lately been sitting out of public life. In October, she was diagnosed with a brain tumor as volunteers were circulating petitions for her and supporters were raising money.

“Once I found out I had a brain tumor,” said Lewis, a red beret on her head, a red velvet scarf tossed over a black suit, “I went immediately to have the surgery. I was told that the surgery got 97.97 percent. The good news about that is again, 97.97 percent. The problem with tumors is you don’t know what’s left microscopically.

The former chemistry teacher then had radiation and chemotherapy for six weeks.

“So that was harsh. It’s harsh. But again, there were people there so much worse,” Lewis said. She wasn’t ever in any pain.

“When you’re not in pain and you don’t feel impaired and you feel like you’re still you, it doesn’t feel like such a burden, but it’s scary. Somebody says ‘brain tumor’ and we have . . . all these ideas about brain tumors,” Lewis said. “But when people see me and they talk to me, they’re always relieved.”

Lewis’ meanest critics who mocked her weight may not recognize her now. After weight-loss surgery in March, she was already down 100 pounds by October. She has since lost much more and said she enjoyed buying new clothes in regular stores rather than in the plus-size department.

For the next year, Lewis will have regimented chemo for five days at a time, and then none for the next 23.

This week, she said, is the second with no chemo, so it’s a good week. She said doctors can’t put a “stage” on her tumor without doing more homework to figure that out. She won’t discuss more details about that.

Meanwhile, Lewis has returned to work.

She’s taking back some of the responsibilities that CTU Vice President Jesse Sharkey assumed while she was “on vacation.”

The 26,000-member union’s contract, which was hammered out during the 2012 strike, expires in June. The CTU has begun negotiating with Chicago Public Schools.

The CTU has thrown manpower and money into aldermanic races, too, especially Tara Stamps, a teacher trying to win the 37th Ward, and Susan Garza, a school counselor running in the 10th Ward.

Lewis is campaigning for Jesus “Chuy” Garcia, appearing with him last Saturday at a South Side rally.

“He’s very laid-back because he’s getting things done, solving problems, not screaming and hollering at people,” she said, with a jab at Emanuel. “And I think people mistake that kindness for weakness.”

She also has a new governor to meet, one who likes charter schools more than labor unions, and a new standardized test to fight alongside CPS CEO Barbara Byrd-Bennett.

And on Monday, she’ll address the City Club of Chicago.

“I’m listening to my body and I’m being very respectful of it. So when I’m tired, I’m tired. When I’ve got the energy, I’ve got the energy,” Lewis said.

“The key becomes, how do we transition back to go-go-go? I don’t know that I’ll ever be back to go-go-go. It doesn’t matter. What matters is the work.

“I’m willing to work as hard as I can and give it my all to build the foundation, to make sure we have a very strong foundation of people from all over this city who want to work together to make it better.”

Lewis, famous for her wit, speaks a bit more slowly than she used to. She doesn’t walk as fast either.

Her jokes still come quickly. So do impressions of people who called her while she was really ill: CPS Chief Administrator Tim Cawley, asking if she still wanted Catholic prayers. Emanuel texting her in the hospital to make sure she was OK. Not recognizing the number, she asked who it was.

She said Emanuel’s messages, confirmed by his spokeswoman, surprised her. They hadn’t really talked since he famously yelled at her early in his first term. And aside from a text she sent him after his son was mugged, they haven’t since.

“I haven’t had a chance to talk to him,” Lewis said. “I don’t really have anything to say to him right now. ’Cause it can’t be ‘Good luck,’ ’cause I want him to lose.”

In an email, Emanuel campaign spokesman Steve Mayberry defended the mayor’s track record in neighborhoods, saying that “the mayor is proud of the work he’s done in all corners of the city…”

What if Emanuel prevails?

Lewis sighed.

“What do I do if Rahm wins? I don’t know. What I’ve always been doing. . . . We figured out a way to work with his people, and I guess we’ll figure out a way to work with them again,” she said, “but I don’t really want to hear that.”