The Supreme Court on Monday declined to consider a third request from a group of states to overturn a sweeping Obama administration air pollution rule.

The states, led by Michigan and joined by numerous business and energy interests, told the court that the Environmental Protection Agency’s (EPA) 2012 mercury and air toxics standards rule for coal-fired power plants is illegal, and a lower court erred when it refused to overturn the regulation.

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The Supreme Court ruled nearly a year ago that the EPA did not properly consider costs before writing the regulation. But it did not overturn the rule, instead kicking it down to the Court of Appeals for the District of Columbia Circuit, which decided in December to let the EPA keep enforcing the measure.

The mercury rule is separate from the Clean Power Plan, which limits carbon dioxide emissions and is currently on hold due to a Supreme Court stay.

Justices, as usual, did not say why they declined to take the mercury rule appeal.

The Supreme Court’s Monday decision is a major victory for the Obama administration and environmentalists.

“Today, millions of American families and children can breathe easier knowing that these life-saving limits on toxic pollution are intact,” Vicki Patton, general counsel at the Environmental Defense Fund, said in a statement.

The EPA said it is “pleased” with the decision.

“These practical and achievable standards cut harmful pollution from power plants, saving thousands of lives each year and preventing heart and asthma attacks,” EPA spokeswoman Melissa Harrison said in a statement.

“Power plants are the largest source of mercury in the United States. Mercury is a neurotoxin that can damage children’s developing nervous systems, reducing their ability to think and learn.”

At a $9.6 billion projected annual cost, the mercury rule is one of the most expensive from the Obama administration. It’s been blamed for the shutdowns of hundreds of coal-fired power plants.

But it’s projected to bring between $37 billion and $90 billion in benefits and prevent up to 11,000 premature deaths and 130,000 asthma cases annually.

Andrea Bitely, spokeswoman for Michigan Attorney General Bill Schuette, pointed to the Supreme Court ruling last year that the mercury rule was illegal.

“The EPA blatantly refused to follow that ruling, requiring us to return to the [Supreme Court],” she said. “We are very disappointed in the post-[Justice Antonin] Scalia Court’s decision this year to not enforce Michigan’s victory.”

Michigan and its allies also tried earlier this year to get the Supreme Court to issue a temporary stay on the regulation, which the court refused to do.

The EPA issued a regulation in April to fix the problem identified by the Supreme Court last year, declaring that the cost-benefit analysis for the rule makes it necessary and appropriate.

The litigations that previously challenged the regulation in court are also challenging the April supplement.

— This post was updated at 4:25 p.m.