The U.S. Supreme Court heard oral arguments from the National Mining Association (NMA), 21 states and the Utility Air Regulatory Group challenging the legality of the Environmental Protection Agency’s (EPA) 2012 Mercury and Air Toxics (MATS) rule. The MATS rule, among the costliest in regulatory history, is responsible for retiring up to 20% of the nation’s coal fleet of power plants, according to the NMA.

NMA specifically challenges the failure of the EPA to meet the Clean Air Act’s (CAA) test of “appropriate and necessary” by ignoring costs in promulgating a rule that delivers no more than $6 million of benefits at a cost of $9.6 billion, the NMA said. By omitting cost considerations, the EPA could not meet the test required in the CAA and ignores Congress’s special requirement for regulating power plants as distinct from other sources. By violating the plain meaning of the act, said NMA, the EPA has engineered massive and unanticipated fuel switching that has weakened the reliability of the national grid, deprived the nation of fuel diversity and accelerated unemployment—all for virtually no net environmental benefit.

The U.S. Court of Appeals for D.C. had earlier upheld the EPA’s decision not to weigh costs, but a dissent by Judge Brett Kavanaugh arguing that cost-benefit considerations should apply to the rule appeared to have persuaded justices to hear a challenge on this issue. The court heard arguments for 90 minutes and is expected to rule later this spring or early summer. A favorable ruling could save from retirement approximately 24,000-37,000 megawatts of coal-based generation, according to the NMA.