Specifically, the new Trump administration proposal would repeal a 2011 finding made by the E.P.A. that when the federal government regulates toxic pollution such as mercury from coal-fired power plants, it must also, when considering the cost to industry of that rule, take into account the additional health benefits of reducing other pollutants as a side effect of implementing the regulation. Under the mercury program, the economic benefits of those health effects, known as “co-benefits,” helped to provide a legal and economic justification for the cost to industry of the regulation.

For example, as the nation’s power plants have complied with rule by installing technology to reduce emissions of mercury, they also created the side benefit of reducing pollution of soot and nitrogen oxide, pollutants linked to asthma and lung disease.

The Obama administration estimated that it would cost the electric utility industry an estimated $9.6 billion a year to install that mercury control technology, making it the most expensive clean air regulation ever put forth by the federal government. It found that reducing mercury brings up to $6 million annually in health benefits — a high number, but not as high as the cost to industry. However, it further justified the regulation by citing an additional $80 billion in health benefits from the additional reduction in soot and nitrogen oxide that occur as a side effect of controlling mercury.

The new proposal directs the E.P.A. to no longer take into account those “co-benefits” when considering the economic impact of a regulation.

Should the proposal become final, it would mean that the mercury rule would, on paper, incur far greater economic cost than it would provide quantifiable health benefits. The Trump administration would then be legally justified in weakening the rule.

And that change could also give companies like Murray Energy a legal justification to sue for its deletion entirely, while giving the E.P.A. the legal basis to craft weaker pollution regulations that no longer take into account the co-benefits of eliminating additional pollutants.

“This is a sweeping attack on considering the benefits of cutting hazardous pollution from coal plants,” said John Walke, a legal expert on the Clean Air Act with the Natural Resources Defense Council, an advocacy group that expects to take a lead role in the legal effort to uphold the mercury standard. “This is the first legal step toward eliminating the standard entirely.”