It also speaks to the utter lack of substantive debate about the shape of Brexit during the campaign.

Mr. Johnson never tired of promising to “get Brexit done,” but he steered clear of any explanation of how he would do it, or the trade-offs involved.

The few times he did get into the details, he was often wrong. He erroneously claimed, for example, that companies shipping goods from Northern Ireland to a post-Brexit Britain would not have to fill out any additional customs paperwork, when the terms of his withdrawal agreement with the European Union will clearly require it.

Not getting drawn into the details of Brexit was a deliberate strategy that emerged after some internal debate, said Brett O’Donnell, a Republican political consultant who advised the Conservatives during the campaign.

“There was an instinct to relitigate the arguments on Brexit,” said Mr. O’Donnell, who prepared both George W. Bush and Mitt Romney for their presidential debates and coached Mr. Johnson when he led the Leave campaign in 2016.

But Mr. O’Donnell and other senior advisers, including Mr. Johnson’s senior aide, Dominic Cummings, argued that, “It was not about relitigating the arguments over Brexit but channeling people’s frustration that it had not been done yet,” Mr. O’Donnell said.