This is the essence of Mr. Trump: selling himself or his plans by massaging and embellishing facts, or simply making them up and hoping everyone plays along. It is a strategy he developed, quite purposefully, during his days as a brash New York City real estate tycoon, and later relied on as a TV personality. He even has his own term for it: “truthful hyperbole,” which Mr. Trump described in his 1987 book, “The Art of the Deal,” as “an innocent form of exaggeration — and a very effective form of promotion.”

The tactic worked back when he was cajoling banks into giving him giant loans for casinos that would later collapse in bankruptcy, or trying to convince people that his 58-story skyscraper was actually 68 stories, or claiming that his reality show, “The Apprentice,” was the “No. 1 show on television,” when it was in fact 67th. But it’s going to be far harder for him to sustain his illusions now that he leads the most powerful nation on earth and oversees a complex government staffed by millions of people, including some powerful ones who are unlikely to parrot his “facts” when they conflict with the truth. What happens when Mr. Trump, looking through his reality-distortion goggles, claims that America is winning the war against terrorism and his defense secretary, Jim Mattis, surveys the same landscape and disagrees? Which “facts” will the American people be told?

Already, there are signs that Mr. Trump’s closest aides are struggling to cling to some self-respect by edging away from the president’s fantasies. At Tuesday’s White House press briefing, Mr. Spicer defended Mr. Trump’s fraudulent claim that millions of people voted illegally. “He believes what he believes,” Mr. Spicer said. “What does that mean for democracy?” asked a reporter. “It means that I’ve answered your question,” Mr. Spicer replied.

Mr. Trump’s allergy to empirical facts leads naturally to his attacks on the media, whose job it is to report accurately and to hold politicians to account for the things they say and do — goals that are anathema to a huckster. On Thursday, one of Mr. Trump’s top advisers, Stephen Bannon, told The New York Times that the “elite media” is “the opposition party” and should “keep its mouth shut.” Congressional Republicans are getting on board, too: Last week, Representative Lamar Smith of Texas, the chairman of the House Science, Space and Technology Committee, said it was “better to get your news directly from the president. In fact it might be the only way to get the unvarnished truth.”

This attitude is, of course, all too familiar to the citizens of authoritarian regimes around the world — from China to Russia to Turkey to Egypt — where leaders survive by intimidating or imprisoning journalists, writers and artists, or anyone who dares to challenge the “truth” and “information” generated by the regime. Mr. Trump can engage in intimidation through his Twitter and Facebook accounts alone, where he has a direct line to tens of millions of Americans.

A closed disinformation loop may not seem like a big deal when the dispute is over crowd size, but the stakes will soon be far higher. Mr. Trump’s foremost biographer, the investigative journalist Wayne Barrett, who died this month, said in December that “there’s no check on his power except reality.” The nation, and the world, are about to find out if that’s right.