Members of the Supreme Court’s conservative majority seemed inclined on Wednesday to limit or even overturn a key precedent that has empowered executive agencies, threatening regulations in countless areas, including the environment, health care and consumer safety.

Each side warned of devastating consequences should it lose, underscoring how the court’s decision in a highly technical case could reverberate across wide swaths of American life.

Overruling the precedent, Solicitor General Elizabeth B. Prelogar told the justices, would be an “unwarranted shock to the legal system.”

But Justice Brett M. Kavanaugh responded that there were in fact “shocks to the system every four or eight years when a new administration comes in, whether it’s communications law or securities law or competition law or environmental law.”

Judging from questions in two hard-fought arguments that lasted a total of more than three and a half hours, the foundational doctrine of administrative law called Chevron deference appeared to be in peril.

The doctrine takes its name from a 1984 decision, Chevron v. Natural Resources Defense Council, one of the most cited cases in American law. Under it, judges must defer to agencies’ reasonable interpretations of ambiguous statutes. In close cases, and there are many, the views of the agency take priority even if courts might have ruled differently.

Supporters of the doctrine say it allows specialized agencies to fill in gaps in ambiguous statutes to establish uniform rules in their areas of expertise, a practice they say was contemplated by Congress.

Its opponents, including business groups hostile to what they see as overregulation, counter that it is the role of courts, not executive branch officials, to determine the meanings of statutes. They also say that agencies’ interpretations can change with new administrations and put a thumb on the scale in favor of the government even when it is a party to the case.

Some conservative justices said courts must decide what laws mean without giving decisive weight to agencies’ views.

Justice Neil M. Gorsuch, for instance, said he was worried that judges would abdicate their responsibilities “and say, automatically, whatever the agency says wins.”

The court’s three liberal members, by contrast, said agencies were often in a better position than courts to interpret ambiguous statutes in their areas of expertise.

“Agencies know things that courts do not,” Justice Elena Kagan said, “and that’s the basis of Chevron.”

She added that discarding the decision would be a strikingly disruptive move, as there have been 70 Supreme Court decisions relying on Chevron, along with 17,000 in the lower courts.

Justice Ketanji Brown Jackson said Congress had given some policy choices to the agencies. “And my concern,” she said, “is that if we take away something like Chevron, the court will then suddenly become a policymaker.”

The fate of Chevron could turn on the votes of Chief Justice John G. Roberts Jr. and Justice Amy Coney Barrett, members of the court’s conservative wing whose questions were not uniformly hostile to the doctrine.

The tone of argument was lively and light, with smiles and banter among the justices, who remained engaged even as the hours went on.

The cases the justices considered were brought on behalf of two sets of fishermen, one in New Jersey and the other in Rhode Island. They objected to a maritime agency’s interpretation of a 1976 law that requires them to carry observers to gather data to prevent overfishing.

The contested interpretation, set out in a 2020 regulation adopted by the National Marine Fisheries Service, required the fishermen not only to transport the observers but also to pay for them, at a rate of about $700 a day.

The U.S. Court of Appeals for the District of Columbia Circuit rejected a challenge to the regulation in the case from New Jersey, citing Chevron.

“Congress has delegated broad authority to an agency with expertise and experience within a specific industry,” Judge Judith Rogers wrote for the majority, adding that “the court’s review thus is limited to the familiar questions of whether Congress has spoken clearly, and if not, whether the implementing agency’s interpretation is reasonable.”

It was, she wrote. “Although the act may not unambiguously resolve whether the service can require industry-funded monitoring,” she wrote, “the service’s interpretation of the act as allowing it to do so is reasonable.”

A unanimous three-judge panel of the First Circuit said pretty much the same thing in the case from Rhode Island. “At the very least,” Judge William J. Kayatta Jr. wrote for the panel, the agency’s interpretation of the 1976 law was “certainly reasonable.”

Wednesday’s argument featured a host of hypothetical questions. Justice Kagan asked who should decide, for instance, whether a product is a drug or a dietary supplement. The answer, she suggested, was an expert agency.

“It’s best to defer to people who do know, who have had long experience on the ground, who have seen a thousand of these kinds of situations,” she said. “And, you know, judges should know what they don’t know.”

Justice Kagan imagined a new statute addressing artificial intelligence, one that would inevitably have gaps and ambiguities.

“Congress can hardly see a week in the future with respect to this subject, let alone a year or a decade in the future,” she said, adding, “Congress knows that this court and lower courts are not competent with respect to deciding all the questions about A.I. that are going to come up in the future.”

Justice Jackson is recused from the New Jersey case, Loper Bright Enterprises v. Raimondo, No. 22-451, having participated in it as a federal appeals court judge. In an unusual move, the Supreme Court agreed to hear a nearly identical case from Rhode Island, Relentless Inc. v. Department of Commerce, No. 22-1219, five months after it agreed to hear the one from New Jersey.

That may have been a sign that the court wanted to have nine members in place as it considers whether to overturn a major precedent.

Paul D. Clement, a lawyer for the fishermen from New Jersey, said Chevron had made life too easy for Congress, which can enact ambiguous statutes and let agencies sort out what they mean. “Chevron makes it so easy for them not to tackle the hard issues and forge a permanent solution,” he said.

The fishermen are represented by two conservative groups, Cause of Action Institute and the New Civil Liberties Alliance. Both have financial ties to the network of foundations and advocacy organizations funded by Charles Koch, a billionaire who has long supported conservative and libertarian causes.

The justices debated the practical impact of their eventual ruling, expected by June, with some saying that Chevron had already largely fallen out of favor.

“How much of an actual question on the ground is this?” Chief Justice Roberts asked Roman Martinez, a lawyer for the Rhode Island fishermen, noting that the Supreme Court had not decided a case using the doctrine in years.

Mr. Martinez said lower courts continued to decide cases under Chevron, as happened in the cases before the court.

The justices were also concerned about whether a decision overturning the decision would give rise to countless challenges to earlier rulings under the doctrine. “Isn’t the door then open for litigants to come back?” Justice Barrett asked, adding, “Isn’t it inviting a flood of litigation?”

The arguments drew a few dozen demonstrators outside the court, despite the winter chill. Those gathered opposed the court overruling the Chevron doctrine.

After the argument, Meghan Lapp, the fisheries liaison for Seafreeze, a seafood company that operates some of the fishing boats involved in the case, spoke outside the Supreme Court, describing a yearslong attempt to challenge a maritime agency’s regulation.

“Quite frankly, nobody cared,” she said. “The agency is not afraid of us. They’re not afraid of the little guy because they know they get judicial deference.”

She added, “I hope that ends here today.”

Abbie VanSickle contributed reporting.