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.