The regulation would primarily affect the 600 power plants in the United States that are fired by coal, and could ultimately shutter hundreds of them, depending on how it is written.

In anticipation, coal-heavy states are extensively lobbying the environmental agency. John Lyons, Kentucky’s assistant secretary for climate change, said the Natural Resources Defense Council proposal “would shut down our coal-fired generation at a certain point, and that’s just unacceptable.”

Overall, coal supplies about 40 percent of the nation’s electricity, but states like Kentucky, Ohio and Missouri rely on coal for 80 percent to 90 percent of their power.

Mr. Lyons’s reaction underscores a central risk of the regulation: Handing so much choice to the states sets up the likelihood that Republican governors opposing climate policy will fight the federal requirement, either by suing the E.P.A. or by refusing to create plans to carry it out.

E.P.A. officials have also been warily watching the troubled rollout of the Affordable Care Act and the 36 governors who balked at setting up state health care exchanges. People close to the climate regulation process say they view the health care rollout as an object lesson in how they need to ease the public reception of what they hope will be a legally bulletproof regulation. The task of writing that language falls chiefly on the shoulders of Joseph Goffman, the agency’s senior counsel in the office of clean air and a 30-year veteran of Clean Air Act legal battles.

The E.P.A. administrator, Gina McCarthy, is in the meantime traveling across the country to meet with governors, coal industry leaders, energy companies and environmentalists to try to smooth the way politically for the rule. Ms. McCarthy, who once worked for Mitt Romney when he was governor of Massachusetts, is known for her ability to get along with Republican governors as well as for her environmental policy expertise.

Image The E.P.A. administrator, Gina McCarthy, is holding meetings to smooth the way for the rule. Credit... Cliff Owen/Associated Press

Top agency officials, including Ms. McCarthy, have also held public listening sessions in 11 cities, and the agency is bolstering the efforts with an online campaign on Twitter, Facebook and Vine, the video-sharing website.