The Obama administration is toughening rules that force states to cut pollution from coal-burning power plants. (Erin Patrick O'Connor/The Washington Post)

President Obama’s signature effort to combat global warming will be in the hands of federal judges this week, as an appeals court in Washington weighs the legality of the administration’s plan to force sharp cuts in power plants’ carbon emissions and push the nation toward cleaner energy sources.

Even after a marathon hearing Tuesday, the legal questions about the Clean Power Plan are almost certain to remain unresolved when Obama leaves office. But the outcome of the case ultimately could shape the president’s environmental legacy and influence how millions of Americans get their electricity.

“It’s the big kahuna,” said David Doniger, a senior attorney for the Natural Resources Defense Council, which backs the proposal.

The sprawling, unpredictable legal battle — which has attracted attention from the Supreme Court — pits the nation’s leading environmental groups, climate scientists and even tech giants such as Apple against more than two dozen states, industry groups and conservative lawmakers.

At issue for the U.S. Court of Appeals for the District of Columbia Circuit is the administration’s ambitious effort to require every state to reduce carbon-dioxide emissions from such sources as coal-fired power plants. The goal is to cut carbon pollution by about one-third by 2030, compared with 2005 levels. States and utility companies could meet that goal by expanding investment in solar and wind power or nuclear energy, among other actions.

The administration has put in place new regulations to curb car and truck emissions and to limit the amount of mercury and methane released into the atmosphere. It also has taken steps toward regulating airplane emissions.

But the Clean Power Plan, which represents the most significant attempt yet to control CO2 emissions as a pollutant, is the centerpiece of the administration’s efforts to slash the greenhouse-gas emissions that scientists agree are fueling the planet’s rapid warming. It is an integral part of the commitment U.S. officials made as part of a historic climate accord signed in Paris in December, in which countries around the world vowed to cut emissions aggressively in coming years.

Question of authority

Activists and other supporters insist that the Environmental Protection Agency is acting within its authority to try to reduce the public health threat posed by CO2 pollution. In court filings, the groups say the agency responded to concerns from industry and state officials by ensuring flexibility and a gradual phase-in of the new rules.

Well before the plan was finalized, they note, companies were working to generate cleaner electricity and moderate demand. Many utilities were closing old coal-fired plants, not just because of their age but because they would need to comply with new regulations on mercury pollution.

In fact, many states, including some of those challenging the plan, already are on pace to meet its early targets. “Ironically, even if it’s struck down, we may see a reduction in greenhouse-gas emissions in the electricity sector because of trends in moving to natural gas,” said Ann E. Carlson, an environmental law professor at the University of California at Los Angeles. But, she added, “What the court has to say about EPA’s authority . . . will be important for future administrations. It will have consequences.”

The agency and its backers argue that existing case law gives it clear authority to regulate carbon pollution.

The Cardinal Power Station, a coal-fired energy plant in Brilliant, Ohio, shown in February. (Michael S. Williamson/The Washington Post)

“These readily achievable reductions are not too much to ask of an industry that contributes disproportionately to a grave public hazard,” according to a brief filed by environmental and public health organizations.

EPA Administrator Gina McCarthy repeatedly has expressed confidence that the courts will uphold the plan. In a statement, she said it “is rooted in sound science, implements authorities expressly codified in our laws and is broadly supported by the American people.”

But 27 state attorneys general, joined by some coal companies, utilities and a lengthy list of Republican lawmakers led by Senate Majority Leader Mitch McConnell (Ky.), see the issue in starkly different terms. They want the court to scrap the plan, saying the EPA is overstepping its legal authority, in part by going further than Congress intended in the Clean Air Act.

“Regardless of the importance of the global issue EPA seeks to address, it may not usurp lawmaking authority that belongs to Congress or judicial power that belongs to the courts,” according to the brief filed by a coalition of power companies. Representing them in court Tuesday will be constitutional law scholar Laurence H. Tribe, a Harvard Law professor and former mentor to Obama.

Colorado Attorney General Cynthia H. Coffman (R) said there is “a misconception” that states challenging the Clean Power Plan are somehow against clean air and renewable energy.

“That’s certainly not the case. That’s not what the fight is about,” she said. “It’s a fight about the ability of the states to control their own destiny.”

Oklahoma Attorney General E. Scott Pruitt (R), who also is challenging the plan, agreed that the importance of regulating emissions is not the central question in the case.

“What concerns the states is the process, the procedure, the authority that the EPA is exerting that we think is entirely inconsistent with its constitutional and statutory authority,” Pruitt said. “They tend to have this approach that the end justifies the means.”

Day-long hearing

The case, West Virginia v. Environmental Protection Agency, has been marked by legal twists and turns.

In February, the Supreme Court froze implementation of the regulations even before the case reached it. The high court’s extremely unusual move, which did not address the merits of the lawsuits against the EPA, came just days before the sudden death of Justice Antonin Scalia.

That 5-to-4 decision left the case to the D.C. Circuit, which broke with its usual process and decided in May to put the issue before a full panel of active judges. They will hear oral arguments in a day-long session involving more than a dozen attorneys.

The outcome could reveal to what extent Obama’s judicial nominations have affected the court’s political dynamics. The 10-judge panel hearing Tuesday’s arguments will include six judges appointed by Democrats, including four by Obama and four appointed by Republicans. (Obama’s Supreme Court nominee, Judge Merrick B. Garland, is not hearing cases while he awaits a Senate vote.)

Despite those numbers, a favorable outcome for the president is not inevitable. In a case that involves questions about executive-branch authority and how much power agencies should have in relation to Congress and the states, judicial philosophies do not necessarily line up with party affiliation.

The appeals court’s effort to fast-track the case is an acknowledgment of its political importance as Obama’s term comes to a close — and a nod to the Supreme Court’s interest in the questions it raises about the reach of presidential power.

But the D.C. Circuit’s decision, which is unlikely to come before a new president is elected Nov. 8, could hold even more weight given the possibility that Scalia’s death could leave the high court deadlocked when it almost inevitably hears the case.

In the final months of his presidency, Obama finds a key part of his environmental legacy left to the courts — and ultimately, his successor. But even though the fate of the Clean Power Plan is beyond his control, Obama has hardly been silent about its importance.

“There is such a thing as being too late when it comes to climate change,” he said last summer in announcing the finalized plan. He talked about the view of Earth from space — a “blue marble,” he called the planet, that “belongs to all of us.”

“I don’t want my grandkids to not be able to swim in Hawaii or climb a mountain or see a glacier because we didn’t do something,” he said. “That would be shameful of us. This is our moment to get this right and leave something better for our kids.”