In wielding executive power in these ways, Mr. Trump would be following in the footsteps of his predecessors. President Bush cited his commander in chief powers in order to justify interrogation, surveillance and detention polices in the wake of Sept. 11. While Mr. Obama has shied away from Mr. Bush’s constitutional arguments, he has interpreted statutes aggressively, while also relying on constitutional authorities, to justify the military intervention in Libya in 2011 and his nonenforcement of immigration laws.

Mr. Trump has expressed impatience with his critics and hinted that he would use federal powers against them. He wouldn’t be able to put someone in jail merely for criticizing him. But he could direct agencies to use their vast regulatory powers against the companies of executives who have displeased him, like Jeff Bezos, for example, the founder of Amazon. Mr. Trump has already hinted that he would go after Amazon for supposed antitrust violations.

He could direct the Department of Justice to investigate his critics by prioritizing categories of crimes they may have committed. Political opponents could be accused of campaign finance law violations. Former government officials, like Hillary Clinton, could be accused of violating secrecy laws. Even if the charges come to nothing, the legal fees for defendants will be hefty.

Mr. Trump could also crack down on journalists who report on national security issues by enforcing federal secrecy laws more aggressively than previous presidents. President Obama received a lot of criticism for prosecuting government employees who leaked secrets, but the Justice Department did not bring charges against the journalists who published the leaked information.

What couldn’t Mr. Trump do? He couldn’t lower (or raise) taxes on his own. He’s supposed to spend funds that Congress appropriates and for the things that Congress appropriates them for — that’s what stands in the way of the wall (unless he persuades Mexico to pay for it and construct it on the other side of the border).

He could not follow through on his promise to impose the death penalty on killers of police officers by executive order. And even where he does act, he needs to make sure his legal theories are in order. If he wanted to withdraw climate regulations because climate change is a hoax perpetuated by China, no court would allow him to. But if he said that the climate regulations were based on a speculative assessment of harms that wouldn’t occur for 100 years, he could succeed.

Much depends on how far Mr. Trump is willing to push existing legal understandings. There is a netherworld of laws that presidents are supposed to comply with but courts don’t enforce. He could send military forces into a foreign country without authorization from Congress; courts would most likely stay out of the dispute. What of his suggestion earlier this year to kill the families of terrorists? Courts typically defer to the executive on matters concerning military activities abroad. He might even try to withhold appropriated funds or shift them around in defiance of Congress’s wishes.