Will “Tariff Man” — President Donald Trump’s nickname for himself — find his tariff powers neutralised by judicial kryptonite? If the sceptical questions from Supreme Court justices in today’s “Liberation Day” tariff case are any guide, Trump’s protectionist superhero act may come crashing down to earth.

At the heart of the case is how to interpret a 1977 law, the International Emergency Economic Powers Act (IEEPA). On 2 April, 2025 — dubbed “Liberation Day” — the Trump administration declared a national emergency and imposed a 10% “baseline” or “global” tariff on imports from nearly every country, citing the IEEPA as its legal basis.

The law empowers the president to “investigate, regulate, direct and compel… any property in which any foreign country has an interest.” The words “tariffs” or “duties” do not appear. The Trump administration points to a precedent set by President Richard Nixon, who in 1971, amid a trade dispute with Japan over currency values, imposed a 10% duty on all imports.

But the IEEPA was enacted in 1977 precisely to cut back presidential power, which Congress at the time thought had been abused by presidents under a preceding law, the Trading with the Enemy Act (1917), which had been passed to regulate commerce with enemy powers during the First World War.

The administration’s legal case that the IEEPA grants presidents effectively unlimited power to impose tariffs, regardless of existing trade treaties and without ratification by Congress, is weak. Already two lower federal courts have rejected its arguments.

In the oral arguments before the Supreme Court on Wednesday, many justices seemed sceptical.  Justice Amy Coney Barrett, a Trump appointee, asked:  “And so is it your contention that every country needed to be tariffed because of threats to the defence and industrial base? I mean, Spain? France?” Justice Gorsuch, another Trump appointee, warned of “a one-way ratchet toward the gradual but continual accretion of power in the executive branch and away from the people’s elected representatives.”

Much of this scepticism stems from the fact that Trump is the first president to invoke the IEEPA in imposing tariffs. According to Trump’s executive order, the emergency required to trigger the IEEPA is the problem of chronic US trade deficits. That diagnosis, while controversial in legal terms, taps into a growing bipartisan belief that America’s long-run prosperity and security have been undermined by decades of deindustrialisation, unfair trade practices by partners such as China, and the offshoring of manufacturing under deals like NAFTA and the WTO.

But even if decades of deindustrialisation are appropriately described as a national emergency, it does not follow that imposing a 10% tariff on all imports and piling on additional, misnamed “reciprocal tariffs” on specific countries that were apparently improvised at the last minute on the basis of a bizarre formula, add up to a rational strategy for reshoring critical American industries.

It is one thing to argue that the US manufacturing and defence base must be protected against the dumping of Chinese steel or autos. It is quite another to argue that national security and American reindustrialisation require tariffs on Canadian dairy or Brazilian coffee. Indeed, the Republican-led Senate has already pushed back, voting to end the national emergency Trump used to impose tariffs on Brazil and passing a resolution against the levies on Canada. Yet both moves are symbolic unless the Republican House follows suit.

In terms of public policy, the best outcome would be for the Court to rule that the president’s authority to impose tariffs under the IEEPA, if it exists at all, should be limited to specific strategic industries or to specific foreign countries.

Even if the Trump administration loses outright, it can impose tariffs on the basis of other laws, which have more qualifications and safeguards, like Section 301 of the Trade Act of 1974 (unfair foreign trade practices) and Section 232 of the Trade Expansion Act of 1962 (national security threats). As the world economy fragments into rival military-industrial blocs, tariffs will be part of the arsenal of every great power. Tariff Man may lose this battle, but win the war.