The E.P.A. put forth the power plants proposal last June, and after taking public comments and revising the plan, the agency is scheduled to reveal it in final form in August. The judges in the United States Court of Appeals for the District of Columbia Circuit rejected the challenge, saying it was unprecedented for a court to review a rule that had been introduced only in the form of a draft.

All three circuit court judges agreed that the challenge was premature.

“Petitioners are champing at the bit to challenge E.P.A.’s anticipated rule restricting carbon dioxide emissions from existing power plants,” Judge Brett Kavanaugh wrote in the opinion. “But E.P.A. has not yet issued a final rule. It has issued only a proposed rule. Petitioners nonetheless ask the court to jump into the fray now. They want us to do something that they candidly acknowledge we have never done before: review the legality of a proposed rule.”

He concluded, “We deny the petitions for review and the petition for a writ of prohibition because the complained-of agency action is not final.”

Liz Purchia, a spokeswoman for the agency, wrote in a statement, “E.P.A. is pleased that the court has denied the challenges to our proposed Clean Power Plan and confirmed our assessment that they are premature.”

Environmentalists also cheered the decision. “The first legal challenge to the Clean Power Plan failed today, and others the polluters will trot out should fail as well,” said David Doniger, the head of the climate and clean air program for the Natural Resources Defense Council, an advocacy group.