The U.S. Supreme Court heard oral arguments Tuesday in a case that could fundamentally alter the legal balance between protecting secrets or protecting whistleblowers.

In 2003, a Federal Air Marshal, Robert MacLean, told an MSNBC reporter that the Transportation Security Administration (TSA) had cancelled missions on commercial flights that required marshals to spend the night away from home. MacLean’s supervisor told him the cancellations were intended to save money and MacLean says he was concerned the cutbacks posed a danger to the public.

When his supervisors traced the MSNBC leak back to MacLean, he was removed from his position for violating a TSA rule prohibiting the disclosure of “sensitive security information,” a category of information below “secret” that is not classified. MacLean argues he was protected by the Whistleblower Protection Act, a 1989 law that shields from retribution employees who publicize information they believe constitutes a “substantial and specific danger to public health or safety.”

The problem for MacLean, and for the courts, is that the WPA has an big exemption in it to prevent leaks that could threaten national security or endanger government officials: the act says it does not protect those who make disclosures “specifically prohibited by law.”

The question before the court Tuesday was what kind of secrets are “specifically prohibited by law”? Can the President and his executive branch agencies create broad categories of non-classified information that can’t be shared with the public by a whistleblower? Or does Congress have to state exactly what kind of information can’t be leaked?

The Supreme Court Justices vigorously debated those fine points for an hour Tuesday morning, carefully picking apart both sides’ arguments, with the liberal justices appearing to sympathize with MacLean and the conservative bloc leaning toward DHS.

Justice Sonia Sotomayor appeared to say at one point that the “facts favored” MacLean’s argument, at least in the context of what entities he leaked the information to. (He told MSNBC and not, say, a foreign government official.) And both Justices Ruth Bader Ginsburg and Stephen Breyer worried that the TSA rule, which states that employees can’t disclose “sensitive security information,” was too broad. How is an employee supposed to know what qualifies as “sensitive security information” and what does not? If you define “sensitive security information” as anything that’s “detrimental to the security of transportation, that seems to me to be everything from a spark plug that’s deficient in an airplane to a terrorist attack,” Breyer said.

But Justices Antonin Scalia and Anthony Kennedy seemed less sympathetic to MacLean’s case. MacLean’s attorney, Neal Katyal, built his argument on the assertion that this TSA rule, though promulgated correctly and at the direction of the U.S. Congress, should not qualify as a “law.” He cited both legislative history and reports from the House and Senate. Indeed, the conference committee report on the WPA does not pull any punches: “The reference to disclosures specifically prohibited by law is meant to refer to statutory law and court interpretations of those statutes,” it reads. “It does not refer to agency rules and regulations.”

While that particular language is pretty stark, several justices seemed unconvinced, noting that conference committee reports are often written by staff members and are not subject to legal scrutiny by members of Congress. “Are you really going to spin out that argument that that is what Congress intended, and what all the members of Congress meant, when they voted on it?” Scalia asked. “I find that hard to believe.”

The Court is likely to release its decision this winter.

Write to Haley Sweetland Edwards at haley.edwards@time.com.