“There has never been, in the history of our Country, a more vicious or hostile Lamestream Media than there is right now, even in the midst of a National Emergency, the Invisible Enemy!” Mr. Trump wrote on Twitter on Monday morning. “FAKE NEWS, THE ENEMY OF THE PEOPLE!” he added.

The first schedule issued by the White House for Monday listed a briefing for 5 p.m., but by late morning it was canceled. Kayleigh McEnany, the newly appointed White House press secretary, told reporters that briefings would probably resume later in the week, perhaps in a different format.

But Mr. Trump hates being seen as managed by his staff, and once he saw some of the television coverage reporting that his own aides thought he should hold fewer briefings, he decided to host one on Monday anyway.

The reversal, in less than two hours, was framed as an announcement of new testing guidance that was actually slated to be put out by lower-level officials. But Mr. Trump decided to bring along corporate executives he had met with just beforehand and have them take the microphone one after another to highlight their efforts to combat the virus.

Some of his allies had been relieved when he passed on any appearances over the weekend and initially canceled Monday’s briefing, hoping that he was coming around to their view that a more disciplined approach would be better. Other Republicans had urged the White House not to have the president brief every day and to limit those sessions he did to more like 30 minutes, which would hone the message and limit the off-script collateral damage.

“Standing at that podium for more than 30 minutes is kind of like being at a bar after 2 a.m.,” said Ari Fleischer, who was a White House press secretary under President George W. Bush. “All the good stuff has probably happened by now and the only thing left is going to be bad. So get out of the bar — or get off the podium after about 30 minutes.”

Mr. Trump ended up spending nearly twice that in the Rose Garden on Monday, but he gave the lectern over to Vice President Mike Pence, Dr. Deborah L. Birx, his pandemic coordinator, and the executives enough that his own time was limited, to the relief of some of his aides and allies.