Perhaps that’s one reason Mr. Johnson has seemed less sure-footed than many analysts expected in the early days of the campaign. A shambling, slipshod figure in the best of times, the prime minister has seemed undisciplined and occasionally uninformed during several recent appearances — lending weight to past criticism that he is often lazy and ill-prepared.

Chatting with manufacturers in Northern Ireland last week, Mr. Johnson offered a rambling defense of the withdrawal agreement he negotiated with the European Union, which appeared to contradict the facts of the deal and caused a minor tempest when a video of the episode surfaced.

Waving a glass as he spoke, Mr. Johnson insisted that companies would not have to fill out extra paperwork when they shipped goods from Northern Ireland to Britain. If they were asked to do so, he said, they should call him “and I will direct them to throw that form in the bin.” But his own government has said that exporters would be required to fill out “exit summary declarations.”

During a visit to the London Electric Vehicle Company in Coventry, England, Mr. Johnson laid out a cogent case for why the Conservatives were the only party that will swiftly exit the European Union. But he stepped on his own climactic line about how he would “turbocharge” the country’s economic future much like the electric vehicles produced in the factory — musing in an aside about whether one could actually turbocharge a battery-operated vehicle.

“This is Boris Johnson and this is what you get — someone who is slightly unprepared, who wings it a bit — and you either like that and think, ‘Oh, this is Boris’ or you think he is appalling for all sorts of reasons,” Mr. Fielding said.

Andrew Gimson, who wrote a biography of Mr. Johnson, took exception to the criticism, saying there was nothing accidental about the prime minister’s antics. They were designed, he said, to keep Mr. Johnson the center of attention, which in turn helps him press his very serious, but potentially tiresome, argument about Brexit.