Mr. Hufbauer’s analysis is already causing waves among policy wonks in Washington and economists on Wall Street.

The easiest promise for Mr. Trump to deliver is the one to renegotiate or leave Nafta, the agreement that abolished tariffs between the United States, Canada and Mexico. That agreement — like nearly all free trade agreements — includes an escape hatch: The United States can withdraw from the pact after giving six months’ notice. Thus Mr. Trump doesn’t need to rip up the agreement, but simply to send Canada and Mexico a letter putting them on notice.

Similar escape hatches would also allow Mr. Trump to withdraw the United States from free trade agreements with other nations, and even from the World Trade Organization. Leaving the World Trade Organization would immediately allow other countries to raise the tariffs they charge on American exports.

How has the White House accumulated such powers? As the United States entered World War I, Congress passed the Trading with the Enemy Act, which gave the president the power to regulate all international trade and financial flows, and even to freeze or seize foreign assets. It was an extremely broad grant of power because the law applies to all international commerce, not just that with enemy nations.

It’s a power meant to be used only in national emergencies or times of war, but this tends to be interpreted very broadly. For instance, the courts allowed President Nixon to add a 10 percent “import surcharge” in 1971, based on the continuing “economic emergency.” Mr. Hufbauer argues that continuing military engagements in Iraq and Afghanistan “would seemingly suffice to satisfy” this requirement.

Alternatively, the president could rely on the International Emergency Economic Powers Act of 1977, originally written to give the president the tools to inflict economic sanctions on America’s enemies. The law could easily be used to restrict trade with just about any country. While the law is supposed to be exercised only in the wake of an “unusual and extraordinary threat,” Mr. Hufbauer says that this is a standard that is easily met, as “the courts have never questioned presidential declarations of a ‘national emergency.’ ”

Beyond these emergency powers, two separate Cold War-era statutes also give the president power to regulate international trade. Under the Trade Expansion Act of 1962, the president can raise tariffs as necessary to strengthen national security. Mr. Trump’s rationale for raising tariffs against Mexico or China probably meet this standard.