If that happens, Scalia could deliver a major blow to Obama's EPA. Scalia is no friend to the administration's regulatory agenda, and he appeared to sympathize with industry challengers fighting to roll back the regulations during oral arguments.
The question is whether EPA violated the law when it decided to regulate toxic air pollution from power plants without first taking into account how much it would cost the industry to comply. Challengers argue that the agency should have factored cost into that determination, while the agency maintains that it was not required to do so under the Clean Air Act.
If the Supreme Court rules against the agency, a variety of outcomes could unfold. The court likely would compel EPA to address cost in its regulatory determination, an outcome that would create uncertainty over the fate of the rule and potentially even delay its finalization until the president leaves office.
"These things take time, and it's already 2015. Depending on how the court rules, I don't know that they could get it done during this administration," said Justin Savage, a former Justice Department environmental lawyer and a partner with the law firm Hogan Lovells.
EPA has consistently maintained that the agency is on firm legal ground with the regulation. But that doesn't mean that supporters of the rule aren't anxious.
A key part of EPA's defense is that the law was silent on whether the agency was required to take cost into account in deciding whether to regulate toxic air pollution from power plants, and that when the law is silent, the agency gets to make the call.
Scalia pushed back against that idea during oral arguments. "I'm not even sure I agree with the premise that when ...ÂÂ Congress says nothing about cost, the agency is entitled to disregard cost," Scalia said, adding: "I would think it's classic arbitrary and capricious agency action for an agency to command something that is outrageously expensive and ...ÂÂ in which the expense vastly exceeds whatever public benefit can be ...Â achieved."
When news broke Monday that Justice Elena Kagan had penned the majority opinion in one of the last cases left standing, it fueled speculation that Scalia could hold sway in the power-plant case. And that that could be bad news for the administration.
"If Scalia is writing the opinion that would be the majority, I can't imagine he would take the side of the EPA," said Brian Potts, a lawyer who represents utilities at the law firm Foley and Lardner.
To be sure, there is no mandate that Scalia will pen the verdict. Another justice could write the ruling. Even if Scalia is charged with shepherding the decision, the court still could rule in favor of the administration. And even if the agency loses, the courts could mandate that the air toxics regulations remain in place until the administration responds to the Supreme Court decision.