But as his campaign played out and his presidency began, Donald J. Trump, the master of the small screen, evolved gradually into a different character, @realdonaldtrump, whose itchy Twitter finger became many things at once: an agenda-setter for the day’s coverage, a weapon against his rivals, a way of firing aides and cabinet secretaries, a grenade he could throw at Republican lawmakers who had crossed him and reporters whose coverage he hated, a window into his psyche, and most of all, an unfiltered pipeline to his supporters.