Scott Pruitt, the head of the Environmental Protection Agency, said Monday that he would issue a formal proposal to repeal the Clean Power Plan, a regulation on power plants that would have reduced domestic demand for coal and curbed the country’s planet-warming emissions.

What is the Clean Power Plan?

The Clean Power Plan was President Barack Obama’s signature policy on climate change, and it represented one of the strongest actions ever taken by the United States to combat global warming. But it has never taken effect, and the Trump administration hopes to repeal it before it does.

Under the rule, which was finalized in 2015, the E.P.A. assigned each state a goal for limiting emissions from existing power plants and gave the states broad latitude in meeting those goals, such as switching from coal to natural gas or building new wind or solar farms. At the time, the agency estimated that the rule would have reduced greenhouse-gas emissions from the power sector 32 percent below 2005 levels by 2030.