England has no chipmunks, so all it takes is the sight of one scampering through the grass for Boris Johnson to flash back to his American childhood:

The treehouse his father built in their Morrison Street yard in Northwest Washington, with its floppy ladder made of garden hoses. The big trees, the neighborhood dogs, the thrill of winning his first prize — a toy dump truck — for some long-forgotten achievement at his Montessori school.

The iconoclastic mayor of London was born in New York when his father was a student, and he lived in America until he was 5. He still holds a U.S. passport, and when he returns to the United States, as he did last week, U.S. immigration authorities at the airport still greet him by saying, “Welcome home.”

People joke that as a 50-year-old natural-born U.S. citizen, Johnson should run for president (forgetting that the Constitution also requires 14 years’ residence here, which he can’t claim).

He certainly has some American political cred. Just this month, he hosted Jeb Bush in London and had dinner with Henry Kissinger in New York. He has just appeared on David Letterman’s fizz-fest (the other guest was It-Girl actress Jennifer Lawrence) and Charlie Rose’s dimly lit chat-scan.

And especially after his high-profile role as toastmaster in chief­ during the 2012 London Olympics, his hair alone, a famously untamed tempest of white-blond thatch, has enviable mane recognition on both sides of the Atlantic.

Boris — he’s become a one-name brand in Britain — has heard the White House joke too many times.

“There is no risk to America,” he says in his deadpan baritone, sitting in a Washington hotel room a few paces from the Capitol dome. “My territorial ambitions do not extend in that direction.”

Fair enough, but it is becoming increasingly clear — blindingly obvious, as the British say — that his ambitions do indeed include the top political prize in the United Kingdom.

Johnson has served as the chief elected official of one of the world’s greatest cities since 2008 and was reelected easily in 2012, his popularity ratings are strong, and even his trademark gaffes — among them getting stuck dangling like a ham on a zip line during the Olympics — tend to make him seem more charming, more Boris.

“If any other politician anywhere in the world was stuck on a zipwire, it would be a disaster,” Prime Minister David Cameron said at the time. “For Boris, it’s an absolute triumph.”

ComRes, a U.K. polling firm, found in a survey this year that Johnson is the most popular politician in Britain, with a 41 percent favorable rating — Cameron was second with 28 — yet nearly the same number agreed that he is “weird.”

Johnson recently announced plans to run in May for Parliament, for a reliably safe Conservative Party seat in a London suburb, where he will instantly be a powerful presence in the House of Commons. Political analysts in London say that it’s not too much of a stretch to see him ultimately succeed his friend Cameron as party leader, making him a serious contender for prime minister.

“Boris is like a great white shark — you can see his outline coming from afar,” says Sonia Purnell, a London author who wrote a biography of Johnson that summed him up as “Blond Ambition.”

Johnson religiously avoids the question, but let’s ask him anyway.

“If we said it was blindingly obvious that you would relish the challenge of being prime minister, it wouldn’t be wrong, would it?”

Johnson sits up in his seat, looking slightly startled, like a polar bear splashed unexpectedly by a cheeky seal.

“If you said it was blindingly obvious that I would relish the challenge,” he says, his voice trailing off. A smile crosses his face, and one of the most gifted orators in a nation of silky speakers starts stumbling along a potholed sentence.

“Um (sigh . . . long pause), uh, I mean . . . (pause) . . . what is certainly true (cough, cough) is that I would relish a Conservative victory at the next elections and, uh (cough, cough) . . . the continued success of my friends.”

All right, then.

Not a declaration of candidacy, exactly, but we hear you, Mr. Mayor.

The outperformer

Johnson was in New York and Washington last week to promote his new book, “The Churchill Factor: How One Man Made History.” The Winston Churchill biography, a U.K. bestseller that is also selling well stateside, is Johnson’s ninth book. The others include a collection of poetry, a satirical novel called “Seventy-Two Virgins,” a treatise on the lessons Europe can learn from the Roman Empire and a book of musings on test-driving some of the world’s most expensive cars.

At least nine books have also been written about Johnson, some of them serious attempts at biography but most fairly naked efforts to cash in on his eccentricities, including “101 Fruitcake Quotes of Boris Johnson,” “The Thinking Man’s Idiot” and one called “Boris Johnson: Everything I Know About the Real World,” which contains nothing but 70 blank pages.

Born Alexander Boris de Pfeffel Johnson and educated at Britain’s fanciest schools, Eton and Oxford, Johnson, like Cameron, is a product of the most upper of upper-crust British society.

He made his name in Britain as a journalist, first at the Daily Telegraph (for which he still writes a weekly column) and then as editor of the Spectator magazine. They are two of Britain’s leading conservative publications. He was then a Conservative Party member of Parliament from 2001 to 2008, when he was elected mayor of London in a race that few expected him to win.

For all his accomplishments, Johnson has always been dogged by a habit of saying outrageous things that can make him sound as tone-deaf as some bumbling 19th-century imperial viceroy. Asked once about the benefits of voting for the Conservative Party, he responded with an “only Boris” joke that charmed his supporters and enraged his detractors: “Voting Tory will cause your wife to have bigger breasts and increase your chances of owning a BMW M3.”

According to the U.K. polling firm YouGov, people who support Johnson describe him as likable, charismatic, intelligent and humorous. Those who don’t call him a buffoon, clownish, arrogant and pompous.

“Someone’s got to stick up for buffoons, they’re a large constituency,” Johnson jokes in the hotel room interview.

Yet despite it all, when London voters had the chance in 2008, they eagerly handed him the keys to City Hall. Now, six years later, all eyes are on his next moves.

A YouGov poll in October asked voters whether they would rather have a drink with Johnson or Cameron, and Johnson won 57 percent to 6. On the plus side for Cameron, those polled said that he would be a far better chief executive of a company and a much better babysitter for their children.

“In many ways, Boris is the opposite of Barack Obama,” said Tony Travers, a professor at the London School of Economics and Political Science. “Obama came in with so much expectation, he could only ever fail. Everybody thought Boris would be a disaster, and he has outperformed all expectations.”

A bit of Churchill

According to Johnson, the Churchill estate approached him to write a book about the wartime prime minister to mark the 50th anniversary of his death in January 2015. Johnson jumped at the chance to write about a man he calls “our greatest 20th-century­ hero.”

Working early mornings and weekends, and avoiding dinner parties for a year, he wrote a 390-page book whose central theme is that a great leader can single-handedly alter the course of history. It’s filled with vivid observations that are easy to imagine in Johnson’s voice, such as when he says that Churchill “had one of those bouncy-castle personalities that starts filling the room and pressing everyone else against the wall.”

It also notes that Churchill had many other qualities that sound familiar: He “was funny, and irreverent, and . . . even by the standards of his time he was politically incorrect,” and he “was eccentric, over the top, camp, with his own special trademark clothes — and a thorough-going genius.”

“I don’t have my own special trademark clothes!” Johnson bellows in his familiar basso profondo when that passage is read to him. “Nor do I claim to be a thorough-going­ genius!”

To be completely fair, Johnson is not equating himself with the Great Winston Churchill, whose name inspired thousands of young Winstons, including John Winston Lennon, born in 1940.

“Lots of slightly corpulent, middle-aged­ British politicians pathetically imagine that they might have some Churchillian quality or other — whether it’s a tendency to weep or a tendency to drink too much,” Johnson says. “What I feel, if anything, is a sense of ever-so-slight depression at the scale of the distance” between him and Churchill. “That’s the right thing to feel.”

The point Johnson seems to be making is that Churchill’s success suggests that it is okay to be Boris — irreverent, maddeningly un-PC and a little undisciplined — and still be an effective leader.

Johnson quotes Churchill’s critics as saying that he had an “inappropriate degree of levity” and “lacks the vital note of sincerity for which the country listens.”

“He was said to crack far too many jokes,” Johnson says. “And yet he did use humor as a weapon to get people’s attention. Most political discourse is numbing. If you are going to engage people, you have to engage them at all levels.”

Johnson is still well known for his witty appearances on a BBC television quiz show called “Have I Got News for You.” When he decided to run for public office, many Brits had a hard time imagining his serious side. You may worship Monty Python, but would you really want those guys in government?

For all his quips in an interview, Johnson is just as likely to start wonking on about “fiscal autonomy” or investment in high-tech incubators.

“If you want other people to take you seriously, you’ve got to take yourself seriously,” he says. “Politics is a very difficult business, and entrusting people with large sums of public money is difficult and requires a great deal of responsibility. So people are entitled to expect a great deal of seriousness.”

While humor is an effective tool, Johnson says, the trick is knowing when to turn it off. He levels his gaze and says: “Sometimes you must have the wit to be dull. And not only have the wit to be dull, but the guts to be dull.”

Top form

It’s Saturday night at Politics and Prose, the upper Northwest Washington bookstore just a few blocks from Johnson’s childhood home on Morrison Street, and more than 200 people have crammed into the shop to hear Johnson speak about Churchill.

The crowd includes many folks old enough to remember listening to the wartime prime minister on the radio, including one older man who holds up a cassette recorder and tapes the entire event.

Johnson is nursing a cold after a week of speeches and interviews, but he is in top form. He thrusts his right hand forward to make a point, then runs it through his hair, leaving it even more spectacularly mussed.

He tells a story: When Churchill was trying to persuade President Franklin D. Roosevelt to join the war in Europe, an aide advised him to “kiss Uncle Sam on both cheeks.” Churchill replied, Johnson says with great dramatic flair, “Yes, but not on all four.”

The crowd roars.

Afterward, Sarah Legowski, 31, who teaches Latin at a D.C. high school, waits in the long line to get Johnson’s signature and a photo. Asked whether she thinks that Johnson’s brand of politics would play in the United States, she says, “God, no!”

“American politics tends to drum the amusing out of people,” she says. “It’s something that the Brits can get away with that we can’t.”

Karla Adam in London contributed to this report.