For the past month, Mary Nichols has been the front-runner to become the next administrator of the Environmental Protection Agency.

Dubbed “the queen of green,” Nichols has been appointed head of the powerful California Air Resources Board (CARB) four times by three different California governors from two different parties. She has pressed auto companies to boost fuel efficiency, taken on oil companies, and capped pollution from power plants and industry. Recently she fought the Trump administration to defend California’s right to set its own emissions standards.

But environmental justice activists from her own state are hoping to derail Nichols’s candidacy, lobbying the Biden transition team to choose among several candidates of color.

The fight has exposed divisions within the Democratic Party over the best way to tackle two of the biggest issues facing President-elect Joe Biden: racial equity and climate change.

In a tweet over the weekend, Friends of the Earth (Action) said, “Joe Biden has stated that he is committed to environmental justice. He must now walk the walk by NOT nominating Mary Nichols — who has a bleak track record in addressing environmental racism — to head EPA.”

In reply, liberal Rep. Jared Huffman (D-Calif.) replied on Twitter: “Strongly and totally disagree with my FOE friends on this. Mary is a climate champion almost without peer. She has solid progressive values and is uniquely talented and qualified to lead EPA.”

One flash point has been one of Nichols’s greatest achievements: a cap-and-trade program for greenhouse gas emissions that has made California a model for other states and nations. The program limits the amount of carbon dioxide, methane and other greenhouse gases that businesses can emit but gives them flexibility over ways to meet the cap. Companies can offset excess emissions in one place by paying for forestation and other projects that reduce those gases elsewhere. The cap grows tighter over time.

Nichols and others say cap-and-trade is a powerful economic tool to curb the pollution driving climate change. Fifty-seven percent of the revenue collected from cap-and-trade credits, or $3.5 billion, has been spent on disadvantaged and low-income communities most affected by industrial emissions — substantially more than the 35 percent required by state law, according to CARB.

But environmental justice activists say that cap-and-trade allows companies to continue polluting, albeit on a declining basis, and that “fenceline communities” — poor neighborhoods and communities of color that tend to be located near industry — bear the burden.

Activists on the left have argued that there are several candidates of color who deserve the top spot at the EPA. The Sunrise Movement has endorsed two former senior EPA officials who served under President Barack Obama — Heather McTeer Toney and Mustafa Santiago Ali — as well as Rep. A. Donald McEachin (D-Va.), all of whom are Black. Kevin de León, a Latino Los Angeles City Council member and former president of the California Senate, also has advocates. The Congressional Hispanic Caucus has submitted a list that includes Richard Revesz, who is Argentine American and directs New York University School of Law’s Institute for Policy Integrity.

Gladys Limón, executive director of California Environmental Justice Alliance and CEJA Action, was among about 70 activists who shared concerns about Nichols in a Dec. 2 letter to the Biden transition team.

“We can and must achieve our climate goals by both protecting the health and safety of front-line communities and decreasing our overall greenhouse gas emissions,” Limón said in an interview. Noting that there were “eminently qualified” people of color, she said, “We need a transformative leader.”

Many who have worked with Nichols over her 50-year-long career say she is that kind of leader.

“Mary Nichols is brilliant,” former GOP governor Arnold Schwarzenegger told radio host and Washington Post contributing columnist Hugh Hewitt last week. The former governor, who picked Nichols to run CARB in 2007, said, “I think she would really be great, and she will be able to be a person that will work with the car companies and work with the fossil fuel companies, and to do it in a sensible way.”

In a key endorsement, Senate Minority Leader Charles E. Schumer (D-N.Y.) has asked the Biden transition team to pick Nichols. Schumer is a proponent of a cash-for-clunkers program like California’s to encourage drivers to trade in gas guzzlers for electric and other low-emissions vehicles.

Most of the nation’s biggest environmental groups and their staffers did not sign the letter sent to the Biden team.

“The environmental justice community is rightly concerned about market-based approaches,” said Natural Resources Defense Council President Gina McCarthy, who headed the EPA during Obama’s second term.

But Nichols has a “lifetime of commitment” to the environment and resisted Trump administration efforts to roll back environmental protections.

“California has kept the U.S. in the action and moved the envelope on most things, most definitely the car work,” McCarthy said.

Nichols often describes arriving in Los Angeles for the first time at the end of a 1969 cross-country road trip and being struck by the smog with the peculiar tint that blanketed the basin. She called it “Day-Glo orange,” and it was harming the health of Californians.

Three years later, fresh out of Yale Law School, Nichols became the first person to sue the government under the Clean Air Act, demanding that the administration of then-Gov. Ronald Reagan (R) meet pollution limits under the newly created EPA. Nichols won.

Nearly five decades later, if she gets a chance to run the EPA, Nichols said in an interview with The Post, her “number one priority” would be to rebuild an agency that has lost about 1 in 6 employees in research and development during the Trump administration.

“The single greatest assault on the agency’s mission has been the mistreatment of science and scientists by the agency,” she said. “You’d start off by bringing back many people who have been let go over the last four years or who chose to leave voluntarily rather than being involved in the anti-science the agency has been engaged in.”

It is difficult to imagine a bigger change in the EPA’s leadership. Exit the former coal lobbyist Andrew Wheeler, who is serving under President Trump. Enter Nichols, who has spent her life tackling smog and greenhouse gas emissions, and who as a senior EPA official under President Bill Clinton helped to draft the first national standards for dangerous fine-particle air pollution and to oversee the highly successful acid rain trading program.

The Clean Air Act “was and I think it still is the most powerful action-forcing piece of legislation that the Congress has ever passed,” Nichols said in October ahead of the 50th anniversary of the EPA.

In large part thanks to Nichols’s use of the Clean Air Act, the number of smog alert days in California plunged from 186 in 1967 to zero in 2019. And the state’s annual greenhouse gas emissions hit their 2020 target four years early, reaching the lower 1990 level.

Few people can rival the experience of Nichols, 75, who was first appointed to CARB by Democratic Gov. Edmund G. “Jerry” Brown in 1975. She was named chair in 1979 and reappointed to the top slot in 2007 and 2011.

The agency, which is responsible for air quality in a state that would have the world’s fifth-largest economy if it were a country, gave her a powerful platform. California’s market muscle made companies eager to comply, even if they preferred lower standards.

Between her stints in state government, she was a senior staff attorney at the Natural Resources Defense Council and, under Clinton, assistant EPA administrator for air and radiation. She plans to step down from CARB at the end of December after 13 consecutive years.

Time magazine in 2013 called her one of the 100 most influential people in the world. Her Twitter handle reads: @AirResources Chair, Angeleno, #ZEV driver. Known as California’s #CleanAir Warrior, air-head & Grandma.”

This year, despite the pandemic, Nichols has continued to press for change. She turned down a request by the trucking association to delay a requirement that an increasing number of trucks sold in California be zero emissions.

The process hasn’t always been smooth. Nichols, in an interview with the Los Angeles Times in 1991, recalled CARB hearings in a Los Angeles building circled by a convoy of gasoline tanker trucks and in a room filled with Hells Angels members protesting motorcycle regulations.

But, Nichols later said in a speech, “the Air Board’s progressively strict vehicle emission standards have spurred major advancements in the design of cleaner engines and fuels nationwide — from catalytic converters to unleaded gasoline and zero-emission electric cars.”

That track record is due in part to good political instincts, said Kate Gordon, who heads the Office of Planning and Research under California Gov. Gavin Newsom (D).

“What makes Mary exceptional is that she’s not just a really smart and really good regulator, she’s also a tactical genius,” Gordon said. “She’s extremely good at recognizing the right time to do certain things and how to move forward.”

To achieve a lasting program, government needs to reach agreement with industry, Nichols said: “Our whole system works by the agreement of the governed. By and large they are willing to go along with the rules, and those who are complying have strong interest in making sure those flouting the rules get caught.”

Still, many businesses see Nichols’s agency as intrusive. CARB has set stricter emissions standards for vehicles, fuels and consumer products. And it has 35 local air pollution control districts that regulate emissions from a wide variety of businesses, including oil refineries, auto body shops and dry cleaners.

“Mary obviously knows the air program backwards and forwards,” said Stephen Brown, a former government relations executive at Tesoro, a California-based oil refiner that is now part of Marathon Petroleum. “Ideologically, she is a true believer but not blind to practical realities which impact deadlines and enforcement decisions.”

Some have asked why Nichols held stock in oil and gas companies, including Occidental Petroleum, while she led the fight against greenhouse gas emissions produced by that industry. She owned smaller stakes in Exxon Mobil, Chevron, Schlumberger and a pair of major pipeline companies including TransCanada, according to required disclosure statements.

“I have been, and am, in compliance with California’s conflict-of-interest laws,” Nichols said, adding that she would fully comply with ethics requirements if she were asked to join the Biden administration.

Over and over again, Nichols has played a role best described as negotiator, who often asked aides for less costly compliance options so strict standards wouldn’t be too onerous.

“She always listened to the regulated industry when people would come in and argue against something,” said a lawyer who worked with her at the EPA but wasn’t authorized to speak publicly, “if for no better reason than to understand whether you could design something a bit smarter to understand their needs — but also to understand what they had to throw at you.”

The auto industry has been at the center of Nichols’s agenda. The industry had long complained about the need to meet two standards, thanks to a federal waiver that gives California the authority to set more stringent air pollution standards than the federal government.

In 2009, the Obama administration decided to do away with the dual standards by raising U.S. federal standards to match California’s. U.S. auto companies, on the ropes because of the financial crisis and receiving federal bailouts, agreed.

But with Trump’s election, upheaval arrived. Nichols, who once drove a Toyota Mirai powered by a hydrogen fuel cell and now drives a Tesla 3, fought with Trump officials over their move to relax fuel-efficiency standards and deny California’s right under the Clean Air Act to set its own tougher limits.

Then, last summer, Nichols and her staff announced they had secretly negotiated an agreement with four automobile companies that would come close to meeting the Obama-era tailpipe standards. The carmakers — Ford Motor, BMW, Volkswagen and Honda Motor — got greater flexibility in achieving a fleetwide average of nearly 50 miles per gallon by model year 2026. The Trump administration’s plan calls for cars and light trucks to average 40.4 miles per gallon by 2026.

Nichols called the deal an “olive branch” that might prevent “massive backsliding.” But the Trump administration went ahead and weakened the Obama-era regulation.

Nichols hasn’t hesitated to hit back at the current EPA administrator, threatening to split the auto market if the agency weakened pollution limits on autos. “Please do your job — or step aside & and let the states lead the way!” she tweeted in October 2019, along with a photo of herself wearing a green button with the slogan, “EPA Get Out of the Way.”

With Biden’s election, GM — which sided with Trump in a lawsuit over California’s air authority — did an about-face. After chief executive Mary Barra phoned Nichols to tell her GM was dropping the suit, Nichols said dryly, “I was pleased to be in communication with Mary Barra again.”

Jeffrey R. Holmstead, a partner at the law firm Bracewell and a former assistant EPA administrator under President George W. Bush, says Nichols could face opposition from Republicans during a Senate confirmation.

“She has been a pretty aggressive supporter of climate change initiatives in California that go far beyond what Senate Republicans would be comfortable with,” he said.

Wheeler has made it personal. In an email sent to lawmakers while Nichols was testifying before Congress last June, he accused her of lying. Earlier he had said that the regulations Nichols pursued were “social engineering.”

“All regulations can be called social engineering,” she said. “As a society we’ve made a decision that we can’t rely on private companies or the market to achieve our goals. But we hope to have regulations that are as market-friendly and consumer-friendly as possible.”