Here are some key exchanges from the session and what they mean:

JUSTICE DEPARTMENT ATTORNEY BRETT SHUMATE: Ieepa, to be sure, has not been used for tariffs…
JUDGE JIMMIE REYNA: That’s what we’re concerned with here. When is the first time that Ieepa was used for tariffs…?
SHUMATE: This is the first time Ieepa has been used for tariffs.
REYNA: And yet the statute’s been in existence for over 50 years, correct?…Why is that? Has there been no national emergencies in 50 years?

Reyna was pushing back on Shumate’s insistence that it is “not unusual” for Ieepa not to specifically mention tariffs. The judge said he thought it was “very unusual in the trade framework” to assume Congress intended to authorize tariffs by implication rather than explicitly. Shumate argued the court should look at it the other way: The statute doesn’t explicitly forbid the president from imposing tariffs, so Trump shouldn’t be seen to have exceeded the powers it grants.

JUDGE LEONARD STARK: Coming back to the language of Ieepa, is it your contention that ‘regulate’ carries with it the plain and ordinary meaning to always include tariffs or taxing?
SHUMATE: The word takes its meaning from context, right? So FDA, when they’re authorized to regulate drugs, they’re not authorized in that situation to impose tariffs. But here, Ieepa says not just regulate, but regulate importation.

Although the 1977 law doesn’t mention tariffs, it does authorize the president to “regulate” a variety of international economic transactions after declaring an emergency. The administration’s argument rests on defining regulate to include imposition of tariffs. In another bad sign for the administration, Judge Alan Lourie said it was significant that other language in the statute didn’t seem to implicate tariffs, citing a legal expression, “You know it by its friends.”

“‘Tariffs’ seems to have no friends in that statute, so why would we read tariffs into that statute?” he said.

JUDGE RAYMOND CHEN: It seems pretty clear to me that ‘Yoshida’ is telling us that no, the president doesn’t have the authority to rewrite the tariff schedules, and it seems like in this case, that’s what the president is trying to do.

Chen and the other judges and lawyers in the case are trying to sift the bounds of presidential power from the complex political and legal events that led to the Ieepa law. In 1971, President Richard Nixon imposed a 10% tariff on imported goods under a different measure, the Trading With the Enemy Act. “Yoshida” is a 1975 court decision that upheld those tariffs but stressed that it found them justified by the particular context.

The Trump administration argues that Ieepa codified for the president the wide tariff power Nixon asserted. Challengers say that, to the contrary, “Yoshida” only approved a limited power to impose tariffs and that Congress diluted the authority Nixon asserted in a series of laws that specified what steps presidents could take in response to particular emergencies.

CHIEF JUDGE KIMBERLY MOORE: If we perceive ourselves as having some ability to review [Trump’s claims of an extraordinary threat], what standard ought we to use for that, and how detailed a review do you think I get to do?

A threshold question in the case involves the president’s discretion to declare an emergency. The administration argues that courts have no power to second-guess Trump when he finds that an extraordinary and unusual threat requires him to set aside normally applicable laws. Historically, courts have been reluctant to do so, relying on the president’s good faith and access to secret information to make determinations that some might find questionable based on publicly available facts. But Trump has been quick to label chronic problems like the trade deficit or familiar events such as protests against government policies as emergencies that trigger extraordinary presidential powers. That freewheeling approach to emergency powers has left some judges reluctant to afford the Trump administration the same deference previous administrations were given.

JUDGE TIMOTHY DYK: It’s just hard for me to see that Congress intended to give the president in Ieepa the wholesale authority to throw out the tariff schedule that Congress had adopted after years of careful work, and revise every one of these tariff rates.

The Constitution assigns to Congress the power to regulate foreign trade and impose duties on imports, and lawmakers have enacted tariff schedules and trade agreements through the legislative process. In modern times, Congress has delegated authority to the executive branch to modify tariffs in certain situations; the Trade Act of 1974, for instance, allows the imposition of tariffs if investigations by the U.S. International Trade Commission or the U.S. trade representative determine unfair practices by trading partners. Those challenging Trump’s tariffs say that Congress already has provided the president methods to impose import taxes in response to urgent developments—and wouldn’t have intended to let the president unilaterally junk the entire complicated trade framework established through the legislative process.