WASHINGTON — Impetuous and instinctive, convinced of broad but hidden plots to undermine him, eager to fight and prone to what an aide called “alternative facts,” President Trump has shown in just days in office that he is like few if any occupants of the White House before him.

He sits in the White House at night, watching television or reading social media, and through Twitter issues instant judgments on what he sees. He channels fringe ideas and gives them as much weight as carefully researched reports. He denigrates the conclusions of intelligence professionals and then later denies having done so. He thrives on conflict and chaos.

For a capital that typically struggles to adjust to the ways of a new president every four or eight years, Mr. Trump has posed a singular challenge. Rarely if ever has a president been as reactive to random inputs as Mr. Trump. Career government officials and members of Congress alike are left to discern policy from random Twitter posts spurred by whatever happened to be on television when the president grabbed the remote control.

While that habit generated conversation and consternation when Mr. Trump was a candidate, he now serves as commander in chief and his 140-character pronouncements carry the power of an Olympian lightning bolt. In the course of 24 hours alone, he threatened to send federal forces into Chicago and vowed to investigate his own false claim that three million to five million votes were cast illegally in November, costing him the popular vote. The trumpet blasts come even as he issues daily executive actions overturning longstanding policies across the board.