The proof is in the unobstructed daily view of the Hollywood sign from Nichols’s own front lawn in the Windsor Square neighborhood here; a few decades ago, she wouldn’t have been able to see it most of the time. Southern California hasn’t had a stage-one ozone alert in 15 years, and just this summer it announced that in 2016, its greenhouse-gas pollution fell below 1990 levels for the first time since peaking in 2004—the equivalent of taking 12 million cars off the road and saving 6 billion gallons of gas a year. (That met the state’s legislatively mandated target of reducing the annual greenhouse totals below 431 metric tons four years ahead of schedule.)

“She’s been at this her entire adult life,” says the former EPA administrator Carol Browner, who hired Nichols to come to Washington in 1993 and worked with her again during the Obama administration to develop the standards that Donald Trump is now trying to roll back. “She has shaped the quality of the air we breathe in this country probably more than anybody. She’s just stayed with it. Now she’s got this huge challenge.”

The 73-year-old Nichols is an unusual combination of ferocious advocate and unfailingly collegial negotiator, not above bringing a Tupperware container of chocolate-chip cookies to lighten the mood at a tense meeting. A compact figure with a short fringe of salt-and-pepper hair, she bears a passing resemblance to the Food Network chef Lidia Bastianich. She’s a serious student of the law and science, but her eyes crinkle easily into a smile.

She also has a vivid and eclectic personal story: the daughter of the onetime democratic-socialist mayor of Ithaca, New York, who performed a gay marriage decades before it was legal anywhere; the widow of a major corporate lawyer who represented ExxonMobil in its long settlement fight over the Exxon Valdez oil spill off Alaska; the former senior lay leader of her local Episcopal parish (where Nat King Cole once sang with the choir), and a devoted music fan who never misses the annual New Orleans Jazz Fest.

When the Trump administration first signaled its intention to reject the Obama-era standards and revoke California’s waiver, Nichols’s boss, Jerry Brown, accused the president of “running a one-man demolition derby on science, the Clean Air Act, and a lot of things we are trying to do.” But Nichols herself is much more diplomatic.

“What I’m seeing is a desire on the part of the White House to put this to bed,” she told me when I caught up with her late last month at the Los Angeles Department of Water and Power’s Cleantech Incubator offices, where she often works when not in Sacramento. She had just returned from her first Washington negotiating session with officials from the White House, the EPA, and the Department of Transportation. “The administration seems to want to find a way to accommodate our need and desire to set our own standards. If they want to give relief to the industry, they’re going to have to find a way that both we and the industry think is realistic. The idea is that we all have to work together.”