Supreme Court Chief Justice John Roberts rejected a plea Thursday to block a contentious air pollution rule for power plants in a big victory for the Obama administration.

Roberts’s order came despite his court’s 5-4 decision last year ruling that the Environmental Protection Agency’s (EPA) regulation, known as the Mercury and Air Toxics Standards, is illegal.

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Michigan led a group of 20 states last month — empowered by the Supreme Court’s recent unprecedented decision to halt the EPA’s carbon dioxide rule for power plants — in asking the court to live up to its ruling last year and block the regulation’s enforcement.

“Unless this court stays or enjoins further operation of the Mercury and Air Toxics rule, this court’s recent decision in Michigan v. EPA will be thwarted,” the states wrote in a Feb. 23 filing with the court.

“A stay or injunction is appropriate because this court has already held that the finding on which the rule rests in unlawful and beyond EPA’s statutory authority.”

The EPA responded that no judicial stay is necessary since it’s working to fix the problem the court identified by next month, and the states would not suffer irreparable harm in that time.

“The requested stay would harm the public interest by undermining reliance interests and the public health and environmental benefits associated with the rule,” the government said. “The application lacks merit and should be denied.”

Roberts acted swiftly, waiting less than a day after the EPA’s response brief to side with the Obama administration. He acted unilaterally, electing to reject the request himself rather than take it to the full court, which may have led to a 4-4 split following Justice Antonin Scalia’s death.

The mercury pollution standards, made final in 2012, are a separate regulation from the more controversial and costly carbon dioxide limits for power plants that are also being litigated in court.

The Supreme Court put an unprecedented halt to the carbon rule, known as the Clean Power Plan, last month by a 5-4 vote, when Roberts chose to let the full court vote on the matter. Thursday’s action by Roberts is completely separate from that case.

EPA spokeswoman Melissa Harrison said on Thursday the agency is “very pleased” with Roberts’s order.

“These practical and achievable standards cut harmful pollution from power plants, saving thousands of lives each year and preventing heart and asthma attacks,” she said, adding that the agency’s calculations show $9 in health benefits for every dollar in compliance costs for the regulation.

Harrison confirmed that the EPA next month plans to finalize a fix to the rule to retroactively apply its cost-benefit analysis in the way the Supreme Court said was necessary.

The court ruled last June that the EPA should have conducted a cost-benefit analysis on the regulation before it even decided to start writing it. The agency did so as part of the regulatory process, but the justices said that was not sufficient.

The Supreme Court did not overturn the rule at the time, and the Court of Appeals for the District of Columbia Circuit said in December that the EPA could keep enforcing it.

Environmental groups were pleased with Roberts’s decision.

“It is time to end this damaging litigation and to protect our children from the poisons in our air and water,” said Vickie Patton, general counsel for the Environmental Defense Fund, which participated in the case to help the EPA, along with other green and health groups and some states.

“The Mercury and Air Toxics Standards are safeguarding millions of American children from poisons such as mercury, arsenic and acid gases,” she said.

Michigan and its allies opposing the rule could still try to seek a full-court review of Roberts’s decision.

—This report was updated at 11:32 a.m.