Aboard his gold-plated jumbo jet, the Republican nominee does not like to rest or be alone with his thoughts, insisting that aides stay up and keep talking to him. He prefers the soothing, whispery voice of his son-in-law.

He requires constant assurance that his candidacy is on track. “Look at that crowd!” he exclaimed a few days ago as he flew across Florida, turning to his young press secretary as a TV tuned to Fox News showed images of what he claimed were thousands of people waiting for him on the ground below.

And he is struggling to suppress his bottomless need for attention. As he stood next to the breakfast buffet at his golf club in Doral, Fla., eyeing a tray of pork sausages, he sought to convey restraint when approached by a reporter for The New York Times.

“I’m on message,” Mr. Trump asserted, with effort. “I’m not playing around. In fact, I’m a little nervous standing here talking to you even for just a minute.”