Ten months in, and the presidency of Donald Trump has acquired a reputation for ineffectiveness. Trump’s attempt to repeal Obamacare has failed three times; he has taunted and alienated some congressional Republicans; he has hemorrhaged senior administration officials while struggling to contain an FBI investigation.

But Trump has found near-total success in a few areas of lawmaking—and few are as expansive, with consequences as long-lasting, as environmental policy. Since January, the Trump administration has tried to dismantle former President Barack Obama’s broad legacy of EPA rules meant to lessen the blow of climate change. It has largely succeeded. This week, Trump’s team opens a new front in that war.

Scott Pruitt, the administrator of the U.S. Environmental Protection Agency, announced Monday that he will fully repeal the Clean Power Plan, the Obama administration’s signature plan to reduce greenhouse-gas emissions from the U.S. electricity sector.

It’s the new administration’s most aggressive attempt to materially transform U.S. climate policy since its departure from the Paris Agreement on climate change in June.

The Clean Power Plan was designed to lower greenhouse-gas emissions from American power plants by 32 percent by 2030, as compared to their historical peak in 2005. It constituted the central legal mechanism to meet the American commitments made under the Paris Agreement.

Speaking in the coal-mining town of Hazard, Kentucky, Pruitt alleged that the Clean Power Plan represented illegal executive overreach. He also linked it to a “war on coal” that he says was waged by the Obama administration.

“The past administration was unapologetic. They were using every bit of power [and] authority to use the EPA to pick winners and losers on how we generate electricity in this country. That is wrong,” he said. Mitch McConnell, the Senate majority leader and a longtime Republican of Kentucky, appeared by his side.

Environmentalists condemned the repeal and promised to fight it in court. Ditching the Clean Power Plan, they said, would further degrade the planet’s climate by leading to the release of more heat-trapping gas into the atmosphere.

Public-health groups, including the American Lung Association, also condemned the planned repeal. Nearly half of Americans already live in counties with unhealthy levels of air pollution, the organization has said, adding that climate change appears to makes conventional air pollutants worse.

“Science shows that warmer temperatures can reduce air quality, due to increases in ozone and particulate matter,” said Laura Anderko, a professor at Georgetown University. Air pollution poses a particular health threat to children, she noted.

The Clean Power Plan was sold in part on its public-health benefits. A 2015 study in Nature Climate Change found that a set of rules similar to the Clean Power Plan would prevent 220 heart attacks and 3,500 premature deaths per year. Pennsylvania, Ohio, and Texas would all see the greatest gains under the simulated plan, each avoiding hundreds of early deaths annually.

But the Clean Power Plan has also long contained a tension inherent to American environmental law. Since 2007, the Supreme Court has held that the EPA has the legal authority to regulate carbon dioxide and other greenhouse gases under the Clean Air Act.

The Obama administration didn’t pursue regulation at first, attempting to pass the Waxman-Markey climate-change bill through Congress. That bill would have allowed companies to bid on the right to emit carbon dioxide into the atmosphere, mirroring previous federal laws to reduce acid rain. But after it failed in the Senate—and Democrats lost control of Congress in 2010—Obama’s EPA turned to the Clean Air Act. Over the course of several years, the agency studied, issued, and revised state-by-state rules guiding how local governments should reduce greenhouse-gas emissions from their power plants. The final version of the Clean Power Plan was released in August 2015, months before the Paris conference on climate change.

Almost immediately after the Clean Power Plan was published, dozens of state attorneys general sued the Obama administration, alleging the rules were illegal. They were led by Pruitt, then the Oklahoma attorney general and the chairman of the Republican Attorneys General Association. Despite the Supreme Court’s finding that the Clean Air Act allowed the EPA to regulate carbon dioxide, they argued that this plan, the Clean Power Plan, was illegal.

Did the Obama rules break the law? We may never know. The D.C. Circuit Court of Appeals was still considering the case on Election Day last year. Trump ordered the EPA to revisit the plan in an omnibus executive order in March.

Due to the Supreme Court’s 2007 ruling, the Trump administration will have to issue a new regulation to replace the Clean Power Plan. But it has great leeway in when it may issue that new rule, and it appears to be taking its time.

Of course, the Clean Power Plan is not the only change convulsing the U.S. power sector. While solar and wind energy are on the rise worldwide, they have mostly displaced nuclear power in the United States. Coal use has indeed been plummeting in recent years, but this has mostly been due to cheap natural gas made available by fracking.

“You want to know how you bring the coal jobs back? Ban fracking,” said Ted Thomas, the chairman of the Arkansas Public Service Commission and a former Republican legislator in the state, in March. “When you talk to utility professionals, there are more of them who think Elvis is still alive than believe that” the Obama administration’s rules are to blame for the decline in coal, he told me then.

Indeed, energy experts say that the United States may meet the ostensible emissions-reduction goals of the Clean Power Plan. The Rhodium Group, an energy consulting firm at the forefront of climate-economic forecasting, projects that the U.S. power sector will fall by between 27 and 35 percent by 2030 without the Clean Power Plan.

But its experts also caution the 32-percent figure was just an estimate, and the real emissions gains of the Clean Power Plan may have been higher than anticipated. Historically, energy forecasts have undershot the rise of solar and renewable energy.

And there are graver implications to the rollback. The United States is not on track to reduce its carbon emissions fast enough to avert catastrophic global warming—even under the most optimistic economic forecasts. A recent study on the world’s carbon budget found that the world would need to reduce its emissions in ways that parallel “the Great Depression, the years following World War II, and during the collapse of the Soviet Union” to stave off the worst of global warming.