Editor's note: This story has been updated with a statement from Texas Attorney General Ken Paxton's office.

The U.S. Supreme Court has denied a request from Texas and 19 other states to block a landmark federal rule requiring power plants to slash emissions of mercury, acid gases and other toxic metals — a setback for Texas Attorney General Ken Paxtonin a case where he saw earlier success. 

Without offering an explanation, Chief Justice John Roberts on Thursday denied the states’ request for a stay on the rules, according to media reports.

That decision followed a Supreme Court ruling last June — hailed by Texas Republicans — that the U.S. Environmental Protection Agency did not properly weigh the cost of compliance for coal-fired power plants against the benefits to public health while setting the new standards.

The June ruling sent the Obama Administration back to the drawing board on the regulations, which had already gone into effect. But it did not halt them.

The states, led by Michigan, had asked the justices to block the rules during the revision process. Roberts said no.   

Texas environmentalists cheered the decision.

“Today’s move by Chief Justice John Roberts was a huge victory for our health and environment, especially here in Texas, where two of five worst polluting power plants are located,” Sara Smith, deputy director of Environment Texas, said in a statement. 

Many companies have already taken steps to comply with the standards, which Paxton and other critics call too costly.

The EPA, which plans to finalize the revised rules in April, said it was 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,” the agency said in a statement. “Power plants are the largest source of mercury in the United States.” 

Paxton's office vowed to continue its challenge.

"We will continue to work with Michigan and 18 other states to vacate this rule, which the Supreme Court already said was unlawful," said spokeswoman Cynthia Meyer.