Smoke no longer rises out of the smokestacks at the coal-fired Castle Gate Power Plant outside Helper, Utah, in 2015. The plant was closed last year in anticipation of new regulations aimed at cutting carbon emissions. (George Frey/Reuters)

President Obama’s signature effort to combat global warming was alternately lauded as a reasonable attempt to move the nation toward cleaner energy sources and faulted as an unconstitutional, job-killing power grab during seven hours of vigorous legal arguments Tuesday.

With the Clean Power Plan’s future on the line, a 10-judge panel of the U.S. Court of Appeals for the District of Columbia Circuit struggled to discern whether the proposal goes too far in trying to compel power plants to cut carbon-dioxide emissions. The Obama administration contends it merely used its authority to nudge the industry in a direction many companies already are headed — toward more renewable sources of energy such as wind and solar.

This central piece of Obama’s environmental legacy, which will probably remain unsettled even after he leaves office in January, aims to slash the greenhouse-gas emissions that scientists agree are fueling the planet’s rapid warming. It also is an integral part of the commitment U.S. officials made as part of a historic climate accord signed in Paris last December, with countries around the world pledging to cut emissions aggressively in coming years.

More than a dozen lawyers lined up before an overflow crowd Tuesday to argue the case, with the nation’s leading environmental groups, climate scientists and tech giants pitted against more than two dozen states, industry groups and conservative lawmakers.

At least two judges on the panel, Brett M. Kavanaugh and Thomas B. Griffith, suggested that federal agencies such as the Environmental Protection Agency must defer to Congress when it comes to enacting policy with major political and economic implications.

“It’s not just a technicality. It’s rooted in the separation of powers,” Kavanaugh said. “Congress should be making the big policy decisions.”

Attorney Eric Hostetler, who represented the EPA, said the regulations are well within the agency’s authority. He called the effort a “proper and sensible” attempt to regulate carbon emissions, with no more overall economic consequences for the industry than some existing pollution regulations.

Judge David S. Tatel appeared to agree. He noted that the regulations would only modestly affect the percentage of U.S. electricity generated by coal through 2030.

Tatel questioned why opponents consider the regulations “transformative,” adding that it seemed the EPA had invoked its long-standing authority.

The regulations would direct every state to form detailed plans to reduce CO2 emissions from such sources as coal-fired power plants, enough to decrease carbon pollution by about one-third by 2030, compared with 2005 levels.

Lawyers representing the oil and gas industry and other opponents said the EPA’s regulations would force power-plant owners to either shut down or essentially subsidize competing clean-energy industries.

Attorney Peter D. Keisler said the EPA’s far-reaching approach is unprecedented and that new regulations were merely “a lever to force” coal companies out of business.

A long line of spectators waited outside the courthouse just blocks from the U.S. Capitol for more than two hours before the arguments began. Among those listening in the wood-paneled courtroom were EPA Administrator Gina McCarthy, White House advisers and U.S. Sen. Edward J. Markey (D-Mass.).

The D.C. Circuit could take months to issue a ruling — which will almost certainly be appealed to the Supreme Court.

The case, West Virginia v. Environmental Protection Agency, already has had an unpredictable legal path. In an unusual move, the Supreme Court blocked the regulations’ implementation in February. Its 5-to-4 decision, which did not address the merits of the lawsuit, came just days before the sudden death of Justice Antonin Scalia.

With the high court potentially divided, the appellate court’s judgment could stand at least until a ninth justice is seated.

The D.C. Circuit also broke with its usual process when it decided in May to put the issue before a full panel of active judges, including six appointed by Democrats and four by Republicans. Obama’s Supreme Court nominee, Chief Judge Merrick Garland, recused himself from the case.

The Supreme Court has already ruled that the Clean Air Act gives the EPA the authority to regulate carbon-dioxide emissions. The question is whether the agency chose to do so in an appropriate and reasonable way in the Clean Power Plan.

Laurence H. Tribe, a Harvard law professor and former teacher of President Obama, argued against the plan on behalf of Peabody Energy, the nation’s largest coal company. Tribe said the EPA is inappropriately stepping in where Congress has failed to act on climate change.

Doing so created fundamental concerns about overreach by the executive branch, he said. “There’s a reason 27 states are on the petitioners’ side and 19 are on the other,” Tribe asserted.

The court also struggled Tuesday with how to settle another critical issue: two versions of a law that appear to conflict. When Congress amended the Clean Air Act in 1990, it passed different versions of the provision at issue — a predicament that leaves the validity of the EPA’s authority with the Clean Power Plan up to the judges.

Opponents say EPA’s rule amounts to duplicative regulation, because the same plants would be regulated twice under different sections of the law.

Supporters say Congress intended to strengthen environmental protections — not exempt an entire industry from regulation under separate Clean Air Act programs.

Judge Cornelia T.L. Pillard questioned opponents about why the EPA could not regulate the same entity for two different types of pollutants at the same time. Prohibiting the agency from doing so, she said, was like preventing the government from telling motorists to “drive on the right side of the road and drive the speed limit.”

More than once, the judges and attorneys seemed frustrated by the inaction of Congress and the ambiguity of its laws. Griffith wondered aloud why the debate over how to best transform the nation’s electricity system was playing out in front of a group of unelected judges instead of on the floor of the Senate.

“That’s the fundamental problem,” he said.

Correction : An earlier version of this story mistakenly attributed a quote about the EPA’s plan to regulate power plant emissions to Judge David S. Tatel of the U.S. Court of Appeals for the D.C. Circuit. It was spoken by Judge Thomas B. Griffith. The story also incorrectly identified EPA Administrator Gina McCarthy’s title. The story has been updated to correct the attribution and the title.