The D.C. Circuit Court of Appeals will hear arguments over the Obama administration’s landmark Clean Power Plan on Tuesday, not only debating its legality but offering a preview of a likely argument in front of the Supreme Court.

The plan, which requires states to cut greenhouse gas emissions by a cumulative 32 percent from 2005 to 2030, is on hold after the Supreme Court issued a stay in February. Challengers, led by West Virginia Attorney General Patrick Morrisey, have said the stay indicates the high court takes their argument seriously.

Opponents of the rule may have been thrown a last-second curveball on Thursday, just five days before the arguments, when it was announced Judge Cornelia Pillard will participate.

The court had initially hinted that Pillard would recuse herself, along with Judge Merrick Garland, because she did not participate in the decision for an en banc review. That would have left five Democratic-appointed judges and four Republican-appointed ones to decide the case.

But without explanation, Thursday’s lineup announcement said Pillard would participate in the argument and the decision.

With Pillard, who is considered supportive of the EPA, Clean Power Plan supporters hope to have a 6-4 advantage. “With Pillard on the court, it increases the chances that it will be upheld pretty substantially,” Brian Potts, an energy and environmental attorney with Perkins Coie, told Morning Consult.

Pillard and Judge Judith Rogers are seen as the most reliably pro-EPA judges, while Judges Karen LeCraft Henderson, Thomas Griffith and Brett Kavanaugh generally side with industry, Potts said.

But observers should keep an eye on how skeptical Judge Sri Srinivasan is. Srinivasan was appointed by President Obama, but he is also known as “an administrative law guru” and was seen as a potential swing vote when there were only nine judges involved, Potts said.

The challenge consists of two main arguments. Both pertain to whether the Clean Air Act, which directs the EPA to require the “best system of emission reduction” in the power sector, justifies the EPA’s requirement that states find ways to cut greenhouse gas emissions.

The opponents’ first argument challenges the EPA’s “generation-shifting” requirement, which instructs states to shift their overall energy systems away from dirtier fuels such as coal toward cleaner ones like wind and solar, or at least natural gas. Opponents argue that the Clean Air Act’s reference to a “system” of reduced emissions is limited to individual power plants. The EPA could, for example, require carbon-capture technology to be used at coal plants, but it can’t require a state to replace one source of energy with another, opponents argue.

Supporters say the word “system” isn’t limited to an individual power plant. They also say seeking improvements at individual plants could counterintuitively end up increasing emissions, rather than decreasing them. A group of electric utilities that support the Clean Power Plan wrote in a brief supporting the plan that such a rule “would increase the cost-competitiveness of those units compared to lower-emitting fossil sources,” leading to the increased use of coal and ultimately increased emissions.

The argument about generation shifting could be particularly interesting to the circuit court, Potts said, because these justices have heard cases over Clean Air Act regulations before, but those cases generally pertained to regulations on specific power plants. This is the first rule that hasn’t been “inside the fence line,” Potts said.

The other major argument against the plan is focused on which section of the Clean Air Act justifies the Clean Power Plan. The plan cites Section 111(d) to regulate greenhouse gas emissions, while also using Section 112 to regulate power plants, an overlap that opponents have argued is forbidden within the Clean Air Act. Supporters disagree, and some have argued that the plan could also have been legally initiated under another of Clean Air Act provision that allows the EPA to regulate pollutants if the rule is part of an international deal like the Paris climate agreement.

The court has allocated 70 minutes for the argument over “generation shifting,” 44 minutes on the Section 111(d) debate, and one hour and 44 minutes for a wide range of other arguments, which could raise unexpected sticking points, Potts said.

“There are a lot of smart lawyers on both sides, and there are 10 very smart D.C. circuit court judges. Put them in a room for a few hours and let them debate the merits of these rules, and you’re going to find something out that’s unusual or new,” he said.

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