Dive Brief:

• Supreme Court Chief Justice John Roberts rejected a petition from a group of 20 states to block enforcement of the U.S. EPA's Mercury and Air Toxics Standards (MATS), a regulatory package aimed at limiting hazardous pollution from coal power plants, The Hill reports. 
• The Supreme Court rejected the MATS rule last year in a 5-4 decision, but a lower court allowed the EPA to continue enforcing the regulations as it fixed the cost calculation problems identified by the high court justices. The states, led by Michigan, appealed back to the Supreme Court, saying they would suffer "irreperable harm" if the EPA is allowed to finalize the rule in April.
• In a Wednesday filing, the EPA argued there is no reason to delay the plan and that doing so would undermine "public health and environmental benefits associated with the rule." Most of the coal power plants affected by the MATS rule have either made the necessary emission control upgrades or have shut down. 

Dive Insight:

Chief Justice John Roberts decided to act unilaterally on the stay petition, The Hill reports, rather than allowing the full court to rule, which could have led to a 4-4 split after the death of conservative Justice Antonin Scalia last month. 

The June 2015 Supreme Court ruling on MATS centered on how the agency determined that the rules were "appropriate and necessary," an initial finding that begins the process of writing a regulation. Plaintiffs argued the agency should have considered the costs of the rules at that point, while the EPA said it factored cost into the regulatory process at multiple points and that it was under no obligation from the Clean Air Act to calculate them for the initial finding. 

The Supreme Court, in an opinion penned by the late Justice Scalia, sided with the plaintiffs, writing that the agency should have factored in costs at the beginning. But the ruling did not question the EPA's ability to regulate mercury and other air toxins. Instead, it kicked the rule back down to the D.C. Circuit Court, which was tasked with deciding how the EPA should proceed. 

In December, the D.C. Circuit ruled that the agency could continue to enforce the MATS rule as it fixed the flaws in the program identified by the Supreme Court. Obama administration lawyers told The Hill the fix would be completed by April 16. 

While a Supreme Court block on a rule it has already considered would have been unusual, critics of the MATS rule may have been emboldened by a recent unexpected ruling on another Clean Air Act regulation — the Clean Power Plan. Last month, the court sent shockwaves through the power sector when it placed a judicial stay on the plan, preventing any enforcement of the carbon standards until all court proceedings are complete. 

The motivation behind that stay may have come in part from the enforcement situation involving MATS. When the Supreme Court heard hearings on the rule last year, the MATS compliance period had already begun and most plants affected by the regulations had either made the necessary upgrades to control emissions or had already shut down. 

About 200 plants — representing roughly 20% of U.S. generating capacity — were granted one-year extensions to the original April 2015 compliance deadline under the rule. If the rule were vacated completely, regulatory lawyers told Utility Dive in June, utilities that had already made billion-dollar investments in pollution control equipment to meet the extended compliance deadline could have to face prudence reviews from state regulators on the expenditures. 

Michigan and the other states opposing the rule can still try to seek a full-court review of Roberts' decision, The Hill reports, but it is unlikely the court would block the rule without his support, as the four liberal-leaning justices would not likely support such a move.