As an official in the Justice Department, I followed in Hamilton’s footsteps, advising that President George W. Bush could take vigorous, perhaps extreme, measures to protect the nation after the Sept. 11 attacks, including invading Afghanistan, opening the Guantánamo detention center and conducting military trials and enhanced interrogation of terrorist leaders. Likewise, I supported President Barack Obama when he drew on this source of constitutional power for drone attacks and foreign electronic surveillance.

But even I have grave concerns about Mr. Trump’s uses of presidential power.

During the campaign, Mr. Trump gave little sign that he understood the constitutional roles of the three branches, as when he promised to appoint justices to the Supreme Court who would investigate Hillary Clinton. (Judge Neil M. Gorsuch will not see this as part of his job description.) In his Inaugural Address, Mr. Trump did not acknowledge that his highest responsibility, as demanded by his oath of office, is to “preserve, protect and defend the Constitution.” Instead, he declared his duty to represent the wishes of the people and end “American carnage,” seemingly without any constitutional restraint.

While my robust vision of the presidency supports some of Mr. Trump’s early executive acts — presidents have the power to terminate international agreements like the Trans-Pacific Partnership, for example — others are more dubious. Take his order to build a wall along the border with Mexico, and his suggestion that he will tax Mexican imports or currency transfers to pay for it. The president has no constitutional authority over border control, which the Supreme Court has long found rests in the hands of Congress. Under Article I of the Constitution, only Congress can fund the construction of a wall, a fence or even a walking path along the border. And the president cannot slap a tax or tariff on Mexican imports without Congress.

Nor can Mr. Trump pull the United States out of Nafta, because Congress made the deal with Mexico and Canada by statute. Presidents have no authority to cancel tariff and trade laws unilaterally.