There are few more transactional figures in British politics than Mr. Johnson. Largely devoid of big ideas or a coherent ideological framework, he has climbed the ranks by combining a quicksilver political expediency with a populist’s instinct for appealing to certain voters. In this campaign, he has repeated the phrase “Get Brexit done” so often that it has become a kind of incantation.

Never mind that Brexit — should Mr. Johnson get it done by the end of next month, as he has pledged — would be just the start of the next phase in the country’s painful divorce from the European Union, which could take many more years.

Mr. Johnson is famous for his shambling manner, a raffish untidiness that extends from his clothes to his personal life. It has been one of the ingredients of his political success, lending him an ordinary-guy appeal that would otherwise come hard to a Greek-poetry-quoting graduate of Eton and Oxford.

In the campaign, however, Mr. Johnson has shown himself to be remarkably disciplined, sticking to his message, despite the best efforts of his opponents to shift the debate to other issues, like health care or crime. He seems less the happy warrior than a hyper-cautious tactician, one who started the campaign with a healthy lead and has been determined not to put it at risk.

“In a sense, you’re getting the truer Boris Johnson, which is not the clownish, cuddly fellow,” said Sonia Purnell, another of Mr. Johnson’s biographers. “He is more ruthless than anyone you or I have ever met.”

Mr. Gimson offered a more generous interpretation. “He’s quite an experienced campaigner,” he said. “Boris actually has thought through his campaign plan, and all the evidence on the ground is that it is working.”