In upending the traditional dynamics of governance, Mr. Trump has made himself the most dominant figure in American life even as polls show that he is also the most unpopular first-year president in modern history. He is testing the proposition that a president can still effectively remake the country without securing or even seeking a broader mandate.

“You’ve got someone who is defining the presidency very differently,” said Michael Beschloss, the presidential historian. “Trump is essentially saying, ‘I’m not going to operate just within the boundaries that the founders might have expected or people might have expected for 200 years. I’m going to operate within the boundaries of what is strictly legal, and I’m going to push those boundaries if I can.’”

Not just push. Mr. Trump has shattered boundaries, at least those his predecessors observed. “Everyone else seemed to play within a certain box,” said William M. Daley, who served two presidents, first as a cabinet secretary under Bill Clinton and then as White House chief of staff under Barack Obama. “But this one is totally outside the box.”

In recent times, most presidents have sought to expand the power of their office, and Mr. Trump has continued that trend. Just as Mr. Obama, frustrated by opposition in Congress, made ambitious use of his executive power, only to be reined in at times by the courts, Mr. Trump has turned to his presidential pen to enact sweeping policies.

But he has bristled at the restraints imposed on the presidency as few have, lashing out at judges, lawmakers, investigators and journalists who anger him and expressing frustration that he is not supposed to use the F.B.I. as he sees fit. His sense of government is not based on coalition building or a balancing act between equal branches. It is one where he deems what is necessary and the system should fall in line.

As he told The New York Times in recent days, he believes he has an “absolute right” to order the Justice Department to open or close investigations into himself or his foes. Some lawyers say he has a point, that the Constitution gives him wide latitude over the executive branch. But since Watergate, at least, no other president would publicly assert such power in such a raw political fashion, and critics have warned that thwarting the special counsel’s Russia investigation could lead to impeachment proceedings.

Talk of “absolute” power and a noted affinity for foreign strongmen have fueled fears of authoritarianism. For the most part, Mr. Trump, with some notable exceptions, has demonstrated more bark than bite. But that bark has become a power unto itself, and the question remains whether he will follow through on his threats in the next stage of his tenure or whether his attacks will prove ultimately self-defeating.