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(WASHINGTON) — The Supreme Court on Wednesday heard vigorous arguments over a 40-year-old precedent that has given broad power to federal agencies in how they regulate the environment, public health and safety, tax collection, stock trading and even prescription drug development.

A pair of cases, among the most consequential of the year, were brought by commercial herring fishermen from New Jersey and Rhode Island who asked the justices to overturn the precedent and invalidate a federal rule that would require them to pay the salaries of government monitors aboard their vessels. They argued Congress never intended them to foot the bill.

The Biden administration, backed by major environmental, civil rights and public health organizations, insisted the rule is a “reasonable” and legal reading of an ambiguous federal law.

In 1984, a high court opinion authored by the late Justice Antonin Scalia said that when federal law is ambiguous on a particular question, judges should defer to a federal agency’s interpretation so long as it is “reasonable.” The approach is called the “Chevron Doctrine” after the case by the same name, Chevron v. National Defense Council.

In the case, environmental groups lost by a unanimous 6-0 vote of the court (three justices didn’t participate in the decisions.) The court’s decisions upheld an industry-favored interpretation of an air pollution regulation.

Several of the court’s conservative justices have long been openly critical of the doctrine, indicating a desire to see it overturned or sharply scaled back.

“How do we know where the line is?” asked Justice Clarence Thomas. “How much deference is too much deference?”

Justice Brett Kavanaugh said excessive deference to agencies has created instability in the law.

“The reality is that Chevron itself ushers in shocks to the system every four or eight years,” he said of potential changes to political party control of regulation-writing after each election. “What happens is massive change,” he said.

Justice Neil Gorsuch said that the doctrine had “exploited against the individual in favor of the government,” giving ordinary Americans and their elected representatives little say in highly consequential matters.

Biden administration Solicitor General Elizabeth Prelogar defended the Chevron Doctrine, warning that overturning precedent would upend thousands of other government regulations across American life and invite a flood of new litigation.

“Stare decisis is part of the rules of the game, too,” Prelogar said, urging the Supreme Court to think twice about upending settled law.

The fishermen, backed by major business and conservative advocacy groups with billions of dollars on the line, argued that the doctrine is “egregiously wrong” and that agencies have too much power over Americans’ lives without the express consent of Congress or the courts.

“Of course, courts should pay special attention to what agencies say, but the agency ultimately has to bring its expertise to bear in a way that’s persuasive,” argued Roman Martinez, an attorney representing the fishermen. “And if the court isn’t persuaded, if the court thinks that the law means X even though the agency thinks the law means Y, then the court needs to go with the best interpretation of the statute.”

The government insists subject-matter experts at federal agencies — with intricate knowledge of things such as endangered fisheries, emissions technology, land-use techniques and medical studies — must have flexibility to set policies when laws passed by Congress don’t provide granular detail about how they are to be implemented.

All three of the court’s liberal justices voiced support for that approach.

“I worry about courts overriding an agency’s policymaking prerogative,” said Justice Ketanji Brown Jackson. “I worry that without Chevron, judges are going to decide.”

Justice Elena Kagan said “the best option is to listen carefully and to defer [to an agency] if it’s interpretation is reasonable.”

“Judges should know what they don’t know,” she said.

Chief Justice John Roberts and Justice Amy Coney Barrett, both members of the court’s conservative wing, were more circumspect in their take.

“At oral arguments, Justice Barrett signaled a possible desire to reach a resolution that refines Chevron rather than overturning it. However, if Supreme Court rules as most expect and kills Chevron, many fear it could throw the modern administrative state into chaos,” said Erin Bryan, an expert in regulatory and compliance law at Dorsey & Whitney LLC. “Members of the conservative majority have previously signaled opposition to Chevron and immediately made their ongoing concerns known.”

The court is expected to hand down a decision in the case by the end of June.

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