WASHINGTON — In his first 100 days in power, President Trump has transformed the nation’s highest office in ways both profound and mundane, pushing traditional boundaries, ignoring longstanding protocol and discarding historical precedents as he reshapes the White House in his own image.

But just as Mr. Trump has changed the presidency, advisers and analysts say it has also changed him. Still a mercurial and easily offended provocateur capable of head-spinning gyrations in policy and politics, Mr. Trump nonetheless at times has adapted his approach to both the job and the momentous challenges it entails.

As Washington pauses to evaluate the opening phase of the Trump presidency, the one thing everyone seems to agree on is that, for better or worse, the capital has headed deep into uncharted territory. On almost every one of these first 100 days, Mr. Trump has done or said something that caused presidential historians and seasoned professionals inside the Beltway to use the phrase “never before.”

He has assumed even more power for the presidency, expanding President Barack Obama’s use of executive orders to offset the inability to pass major legislation and making it more independent of the Washington establishment. He has been more aggressive than any other president in using his authority to undo his predecessor’s legacy, particularly on trade, business regulation and the environment. And he has dominated the national conversation perhaps more thoroughly than any president in a generation.