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.

Immigration has driven Mr. Trump even deeper into the constitutional thickets. Even though his executive order halting immigration from seven Muslim nations makes for bad policy, I believe it falls within the law. But after the order was issued, his adviser Rudolph Giuliani disclosed that Mr. Trump had initially asked for “a Muslim ban,” which would most likely violate the Constitution’s protection for freedom of religion or its prohibition on the state establishment of religion, or both — no mean feat. Had Mr. Trump taken advantage of the resources of the executive branch as a whole, not just a few White House advisers, he would not have rushed out an ill-conceived policy made vulnerable to judicial challenge.

Mr. Trump’s firing of the acting attorney general, Sally Yates, for her stated intention not to defend his immigration policy, also raises concerns.