Michael Bloomberg is many things: former New York City mayor, founder of a financial data company, failed presidential candidate and the 11th richest man in the world.

Since leaving public office 10 years ago, Mr. Bloomberg, 81, has also emerged as perhaps the world’s single largest funder of climate activism, making himself an expensive thorn in the side of the fossil fuel industry. The former mayor says that so far he has spent $500 million in an effort to shut down coal and gas plants. This month he said he planned to spend another $500 million on the effort.

The campaign against coal was largely successful. Coal is dirty and expensive, and Mr. Bloomberg’s money helped retire more than 70 percent of the coal-burning power plants in the country, according to the Sierra Club and other organizations, or about 370.

Now he is going after a more challenging target: new petrochemical plants that make fertilizer, plastics and packaging. It won’t be easy.

In recent years, coal had become an increasingly costly and uneconomical way to generate electricity, making plant closures easier to achieve. But plastics and chemicals face no such economic headwinds. In fact, the petroleum industry sees these industries as its future as cars electrify and the burning of fossil fuels declines, so it is investing heavily.

While the new campaign, called Beyond Petrochemicals, has scored a few wins, the petrochemicals business is booming and highly profitable, and plastics remain cheap and in demand. And the industry is fighting back with its own counter effort: Beyond Bloomberg.

Companies and local economic development groups argue that Mr. Bloomberg’s efforts are a heavy-handed approach to a nuanced problem and that the world needs more products made with petrochemicals, not less. They add that his efforts are costing people jobs and hurting an area badly in need of economic growth.

“Attempts to shut down American chemical manufacturing are a bet against millions of hard-working men and women in our industry,” Chris Jahn, the chief executive of the American Chemistry Council, said in a statement. He added that Mr. Bloomberg’s efforts “would send essential jobs overseas and threaten America’s leadership to innovate and compete with countries like China.”

Petrochemicals remain an essential part of modern life, used to make clothing, cars, electronics, fuel and fertilizer, not to mention solar panels and other equipment needed in the transition to cleaner energy sources. There are no easy substitutes for most of the products, and the heavy global demand means that if chemical and fertilizer plants aren’t built in the United States, many will instead simply be built in other countries that may have weaker regulations to protect workers and the environment. But the petrochemical industry is also a major source of carbon dioxide and other greenhouse gases that are dangerously warming the world.

During a recent interview at the headquarters of Bloomberg LP, the financial data company that has given him an estimated net worth of $96 billion, Mr. Bloomberg said he was trying to help slow climate change, which he regarded as an existential threat to humanity.

“If there’s something that can destroy the Earth and kill all living people, then it’s hard to argue you shouldn’t focus on that,” he said. “I want my kids, your kids, to be able to have a life.”

Mr. Bloomberg began his campaign against fossil fuels before he left public office. In 2011, his foundation, Bloomberg Philanthropies, began funding Beyond Coal, the effort to shut down coal plants across the country being led by the Sierra Club. Tax records show that the foundation has given more than $120 million to the group since 2014 and tens of millions more to other nonprofit organizations, including the League of Conservation Voters, the Energy Foundation and the Southern Environmental Law Center.

Last year, Mr. Bloomberg announced a campaign to block the construction of 120 planned petrochemicals plants, most of them in Texas, Louisiana, Pennsylvania, Ohio and West Virginia, pledging to spend $85 million on the effort.

Mr. Bloomberg said his motivation was twofold. He says he wants to stop the construction of facilities that would otherwise continue heating up the globe. And he knows how harmful they can be for local communities, and particularly for people of color, who tend to live near the plants.

“If what they’re doing is going to kill everyone, I don’t care how valuable what they do is,” he said.

Petrochemicals are fast becoming the largest driver of global oil demand and is a growing consumer of natural gas, according to the International Energy Agency. The industry is already generating more than $580 billion in revenues annually, and that number is expected to grow to more than $1 trillion a year over the next decade.

Mr. Bloomberg’s campaign has already notched at least five significant successes by directing small-scale grants to local activist groups, many of them run by families who live near the proposed plants, as well as multimillion-dollar donations to nonprofit law groups in places like Berkeley, Calif. Together, the activists and lawyers have in recent months helped stop construction of two natural gas facilities, an ethylene plant, a methanol plant and a plastics plant in St. James Parish, La., where Sharon Lavigne, 71, is the face of the resistance.

Ms. Lavigne, who grew up in St. James Parish, is among the most outspoken community activists in a stretch of land between New Orleans and Baton Rouge with many industrial facilities. A few years ago, after watching years of development, she founded Rise St. James, a nonprofit organization that works to block new projects in the area.

The group’s billboard towers above freeways in the area, identifying chemical companies that are seeking to expand their operations. Rise St. James was a lead plaintiff in a lawsuit that last year resulted in a judge effectively blocking construction of a new $9.4 billion facility planned by Formosa Plastics.

“It’s a very expensive fight,” she said. “So that Bloomberg money has helped.”

Mr. Bloomberg and Ms. Lavigne declined to say how much money he had contributed to the effort.

‘A surround-sound strategy’

On a sweltering Thursday in August, Ms. Lavigne set out for a “Toxic Tour.” On these drives around the community, she shows visitors the skyline of smokestacks that now defines a stretch of the Mississippi River that, just a few decades ago, was mostly undeveloped swamp.

There is an Exxon Mobil plant, next to a Marathon Oil facility, which is next to a crude oil pipeline terminal. Across the river is a phosphate factory next to an Occidental Chemical facility, which is beside a Nucor steel plant. More than two dozen major facilities have turned what was once bayou backwaters into what is effectively a city-size industrial park, and companies are angling to build several more plants in the area.

Decades of industrial activity have polluted the soil, air and water. Research by Tulane University has linked the pollution to elevated cancer rates in St. James Parish.

Ms. Lavigne and other local activists have been raising the alarm about industrial pollution for decades to limited effect. But in recent years, an influx of cash from Mr. Bloomberg, as well as his formidable Rolodex, has made them more effective.

They have professionalized their operations, rolling out sophisticated websites, hosting fully catered community meetings and spending big on local advertising. And most importantly, they have hired an army of lawyers that has started a legal onslaught aimed at stymying the petrochemical industry one project at a time.

“We have a surround-sound strategy that is pursuing every single avenue,” said Abigail Dillen, the president of Earthjustice, an environmental law group that received $11 million in funding from Bloomberg Philanthropies in 2021. “We are creating uncertainty for the investors. We’re creating delays that are making the projects more expensive.”

Ms. Dillen and her firm helped win Beyond Petrochemicals’ biggest victory to date, stopping the construction of the Formosa plant. Rise St. James and other community groups pressured the U.S. Army Corps of Engineers to retract an important permit and conduct a more detailed environmental assessment.

Lawyers from Earthjustice and other environmental law groups sued the Louisiana Department of Environmental Quality over the lack of valid air quality permits. Today, after five years of work on the project, it is stalled.

Earthjustice also helped stop the development of a new $2.2 billion facility that was going to be built by South Louisiana Methanol in the area. In that case, residents pressured the Parish council to deny the company’s request that the land be rezoned from residential to industrial, while lawyers sued the Environmental Quality Department to revoke an air pollution permit after the company failed to meet a construction deadline.

But their efforts have angered some residents.

Under the midday sun, Ms. Lavigne and the other activists drove down a one-lane dirt road flanked by sugar cane fields to look at a plot of land where the South Louisiana Methanol complex would have gone. As the group peered through a chain-link fence, a pickup truck sped up the road and blocked the group in. The driver got out and confronted Ms. Lavigne, accusing her of trespassing.

After a brief standoff that eventually involved two local sheriffs, tensions eased and the man in the truck explained to Ms. Lavigne that he was the owner of the land that South Louisiana Methanol had planned to build on, and he was expecting a windfall. But when the project was called off, his land was suddenly worth a fraction of what it had been.

‘Out-of-state elites’

Mr. Bloomberg’s efforts have made him Public Enemy No. 1 in the plastics business.

One Monday in May, the director of government affairs for the American Chemistry Council led a session at the West Virginia Manufacturers Association meeting in Wheeling, W.V. The session was promoted as Beyond Bloomberg and promised to educate attendees about “industry’s effort to counter this attack.”

In another presentation by the American Chemistry Council and the Ohio Chemistry Technology Council, this one at an industry workshop in Columbus in July, the groups warned attendees about the effort. “Beyond Petrochemicals will take the most effective components of Bloomberg’s previous campaigns: channeling millions of dollars toward existing influential and effective actors and leveraging the power of government,” the slide deck read.

Local newspapers in Pennsylvania are also railing against the former mayor. “Butt out, Mr. Bloomberg,” an editorial in Pittsburgh Post-Gazette read, accusing Mr. Bloomberg of “using his wealth to impose his will on the people of the Ohio Valley.”

And in the area around the Louisiana industrial sites, representatives from dozens of oil, gas and chemical companies, including Exxon Mobil, Chevron and Dow, have joined a new group called the Louisiana Industry Sustainability Council.

“Out-of-state elites are sending nearly $100 million to Louisiana to dictate our future and our way of life,” read one of the group’s early organizing documents. “Louisianans are the best people to decide what’s right for Louisiana — not Michael Bloomberg.”

The pushback reflected the inherently undemocratic nature of big philanthropy, said David Callahan, the founder of Inside Philanthropy, an independent news site that covers megadonors. “It goes to the heart of philanthropy’s lack of accountability,” he said. “Because, yeah, who is Mike Bloomberg to show up and start pushing around Louisianans?”

This isn’t the first time people have bristled at being told what to do by Mr. Bloomberg.

As the mayor of New York, he took on sugary soda, trans fats, cigarettes and Styrofoam, using the power of City Hall to curb the use of everyday products he felt were adversely affecting the lives of his constituents, whether they liked it or not.

Mr. Bloomberg is unrepentant. The additional $500 million he has committed will be used to continue his efforts to shut down coal and gas power plants around the country and help promote clean energy projects. And he says he is prepared to spend more to keep fighting the construction of new petrochemical plants.

“Just because it’s good economically for you, if it’s killing people, I don’t think we want to do that,” he said.

He dismissed concerns about his efforts limiting economic growth in areas far from New York. “Human beings are innovative,” he said. “If they can’t make it this way, but there’s a profit to be made, they’ll find another way to make it.”

In fact, he may soon be allocating more money toward his efforts to combat climate change. He has said that he intends to leave his 88 percent stake in Bloomberg LP to his foundation, Bloomberg Philanthropies. At that point, it could either sell the company or take it public, resulting in a windfall that could make it the largest charity in the country, surpassing the Bill and Melinda Gates Foundation with its $67 billion endowment.

But while Mr. Bloomberg may be ready to spend billions on his campaign, the petrochemicals industry isn’t going away. Just five plants have been blocked, and more than 100 are still scheduled for completion soon.