Are President Trump’s “Liberation Day” tariffs on, or are they off? And, more important, will legal challenges to these levies put the brakes on the seizure by presidents of both parties of ever-increasing unilateral power?

Two federal courts recently ruled that Mr. Trump lacks authority to impose them, but a specialized federal court with authority over tariff cases, the Court of Appeals for the Federal Circuit, paused the enforcement of those decisions, ordering lawyers to submit legal briefs before a hearing on Monday. One or both cases is likely to land at the Supreme Court in short order.

Understandably, most of the commentary has focused on the practical ramifications for the president’s trade negotiations and the American economy. But the cases may be even more important for the future of a fundamental component of the Constitution’s architecture: the separation of powers, intended by the founders to prevent any of the government’s three branches from becoming all powerful.

The tariff litigation is shaping up as the biggest separation-of-powers controversy since the “steel seizure” case in 1952. There, President Harry Truman assumed control over the nation’s steel mills to ensure the continued supply of armaments needed for the Korean War. The Supreme Court rebuffed Truman, establishing the principle that, even in an emergency, the president cannot take upon himself powers that are granted neither by the Constitution nor by congressional statute. The case has been cited in court decisions repeatedly since then, and is central to most law school courses on constitutional law.

It is clear that the president has no inherent constitutional authority to set or change tariffs or any other taxes. That authority is expressly given to Congress in the first clause of Article I, Section 8, of the Constitution. And it is also clear that Congress has not expressly delegated any power to the president to impose tariffs on his own say-so.

The question is whether vague language in a 50-year-old statute, the International Economic Emergency Powers Act of 1977, gives the president the tariff-setting power. That act delegates various powers to the president “to deal with any unusual and extraordinary threat” to U.S. national security, foreign policy or economy. The statute makes no mention of tariffs or other taxes, and before Mr. Trump, no president ever interpreted it to include such a power.