Boris Johnson knows he’s often cast as Donald J. Trump’s populist twin, and he puts up a perfunctory protest. Analogies between Brexit, which he championed, and Trump’s MAGA movement are “pretty treacherous,” he said, and the caricature of himself as a louche, shambling, Eton-and-Oxford version of Trump does no favors to the once-and-maybe-future American president.

“I wouldn’t want to damage Donald’s chances by encouraging the idea there’s any comparison with me,” Johnson said, over a platter of half-eaten sandwiches. “I wouldn’t want to burden him with the stigma of association.”

Yet, in an interview on Wednesday to promote his new memoir, “Unleashed,” Johnson often spoke as an across-the-pond surrogate for Trump. Drawing on his encounters with Trump when he was prime minister of Britain, Johnson tried to reassure anxious Americans about what a Trump restoration would look like. In a word: He’s bullish.

“I really don’t think that when it comes to it, he is going to want to begin his next presidency by making the Soviet empire great again,” Johnson said. “This guy wants to make America great again, right — not make Russia great again. I’m fundamentally optimistic about a Trump presidency.”

In the interview, conducted at Rupert Murdoch’s News Building in London (Murdoch’s HarperCollins imprint published the book), Johnson offered a systematic defense of Trump on a full range of issues: Ukraine, the Middle East, China, trade, tax policy, even climate change, where Johnson brushed aside Trump’s mockery of windmills and electric vehicles by noting that Elon Musk, the founder of Tesla, “was bounding right beside him” at a recent campaign rally.

He briefly acknowledged the attack on the United States Capitol by Trump’s supporters — “Clearly, the Jan. 6 thing was terrible” — then pivoted back to his sales pitch.