Yet the E.P.A. move also had its detractors within the industry. The vast majority of utility companies have said the proposed changes are now of little benefit to them, because they have already spent the billions of dollars needed to come into compliance, and have urged the Trump administration to leave the mercury measure in place.

Image Robert E. Murray, the chief executive of Murray Energy. Credit... Joshua Roberts/Reuters

The original rule required power plants to reduce emissions of mercury and other toxic pollutants by more than 90 percent over five years. Mercury is a neurotoxin that can damage the brain and nervous system in young children, leading to lower I.Q. and impaired motor skills. The Obama administration estimated that the measure would prevent 4,700 heart attacks and 130,000 asthma attacks as well as 11,000 premature deaths by also eliminating fine particulate matter linked to those ailments.

Estimates like that, however, are at the heart of the current dispute. Business groups like the Chamber of Commerce and others argue the earlier numbers inflated the benefits of the rule, underestimated costs and improperly justified cutting mercury by relying largely on the benefits of reduced particulate matter.

“For a lot of folks, this was just another example of Obama regulatory overreach,” said Jeffrey R. Holmstead, a partner at the law firm Bracewell who served as E.P.A. air chief under the second President George Bush .

The Obama administration itself had broadly accepted that it is difficult to put a specific dollar-figure on some health benefits, for instance, avoiding lost I.Q. points in infants or other fetal harm that has been linked to pregnant women eating mercury-contaminated fish. For that reason, the original rule argued against using a strict cost-benefit analysis to decide whether the regulation should be imposed, said Joseph Goffman, the executive director of Harvard Law School’s Environmental and Energy Law Program.

The new proposal fundamentally changes that approach. It recognizes that difficult-to-quantify benefits exist, but said the administrator “has concluded that the identification of these benefits is not sufficient, in light of the gross imbalance of monetized costs.”

Ann Weeks, senior counsel for the Clean Air Task Force, an environmental group, criticized the rule as “bean counting ,” and said, “This is not tax law. This is public health benefits. It’s a very different calculus.”