But in his lengthy televised coronavirus news briefings, President Trump offers his own interpretation of the rules: He has told reporters that he would not be interested in wearing a mask unless he deemed it important.

Instead, everyone who comes into close contact with Mr. Trump must now be tested for the virus, including journalists in the White House briefing room, according to White House officials.

“I don’t know, somehow sitting in the Oval Office behind that beautiful Resolute Desk,” the president told reporters on Friday, “I think wearing a face mask as I greet presidents, prime ministers, dictators, kings, queens, I don’t know. Somehow, I don’t see it for myself. I just — I just don’t. Maybe I’ll change my mind, but this will pass, and hopefully it’ll pass very quickly.”

It was far from the first time that the first lady’s message differed from Mr. Trump’s, but some examples have been starker than others.

In August 2017, it was Mrs. Trump who stepped forward before her husband to condemn the deadly white nationalist march in Charlottesville, Va., where one woman was killed by a white supremacist: “Our country encourages freedom of speech, but let’s communicate w/o hate in our hearts,” she tweeted after the protest turned violent. “No good comes from violence.”