“Under this action, no more mercury will be emitted into the air than before,” Andrew R. Wheeler, the E.P.A. administrator said in announcing the rule.

Environmental lawyers and public health leaders called the timing of the final mercury rule, as well as its substance, an attack on air quality.

“What is most disconcerting to me is this administration’s lack of interest in science and, frankly, their lack of concern for our nation’s children,” said Aaron Bernstein, interim director of the Center for Climate, Health and the Global Environment at the Harvard T.H. Chan School of Public Health. “Mercury pollution in the United States damages our children’s brains before they even come into the world, and estimates are that that cost is in the billions of dollars.”

Patrick Parenteau, a professor at the Vermont Law School, noted that in virtually every environmental rollback, Mr. Trump’s E.P.A. has acknowledged in the fine print that enormous increases in health problems and deaths will occur because of increased pollution.

A plan to weaken carbon dioxide emissions at power plants, for example, predicted as many as 1,400 additional premature deaths a year. A draft analysis of the soot policy put forward this week showed that tightening the existing standard by 25 percent could save as many as 12,150 lives a year.

Two people close to the administration said the White House was concerned enough about the public perception of loosening environmental rules during the outbreak that it held the mercury plan for several weeks after it passed a review from the White House Office of Information and Regulatory Affairs. E.P.A. officials assured the White House that the agency was merely responding as required to a 2015 Supreme Court ruling that found it must justify the economic impact of the mercury standards.

The weakening of the mercury rule would be one of the most significant regulatory rollbacks engineered by the Trump administration. The existing federal regulation on mercury pollution, completed in 2012, is the most expensive clean air regulation ever written by the E.P.A.