LONDON — When Boris Johnson walks through the black enameled door of 10 Downing Street on Wednesday, he will fulfill what his biographers describe as his relentless “blond ambition” to follow his hero, Winston Churchill, into Britain’s top job.

He will immediately face the buzz saw of Brexit. And although his supporters hope the charismatic Johnson will rally a divided Parliament and a divided country in a way that Theresa May failed to do, he comes into office as a controversial leader, not especially well liked by most Britons.

Johnson — a bombastic, Latin-quoting Oxford classicist with a mop of intentionally mussed yellow hair — made his name as an over-the-top journalist and a colorful London mayor. He then galvanized the successful Brexit campaign in 2016, winning himself many fans but also many enemies. 

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In the results of a leadership contest announced Tuesday, former foreign secretary Johnson captured 92,153 votes to current foreign secretary Jeremy Hunt’s 46,656 — a decisive victory.

But the vote involved only dues-paying members of the Conservative Party. A mere 139,000 people cast ballots in a country of 66 million. Many ­Britons feel left out at a pivotal moment. On social media, ­#NotMyPM was one of the many Johnson-related hashtags trending. A YouGov survey found that 58 percent of Britons have a negative opinion of Johnson — a ­wicked-high number for a first day on the job.

Opposition Labour Party leader Jeremy Corbyn didn’t let an hour pass before firing his first salvo on Twitter. “Boris Johnson has won the support of fewer than 100,000 unrepresentative Conservative Party members by promising tax cuts for the richest, presenting himself as the bankers’ friend, and pushing for a damaging No Deal Brexit,” he wrote. “But he hasn’t won the support of our country.”

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One person who is a fan of Johnson’s is President Trump. The U.S. leader, who hasn’t been shy about stepping into British domestic affairs, offered the highest praise to Johnson on Tuesday: “He’s tough and he’s smart. They say the British Trump. They say that’s a good thing. They like me over there. That’s what they wanted. He’ll get it done. Boris is good.”

The postwar “special relationship” has had a rocky month. British Ambassador Kim Darroch provoked the president’s ire when leaked diplomatic cables showed him describing Trump as “insecure” and his administration as “dysfunctional.” Trump lashed out. Darroch resigned.

Johnson provides a chance for a reset. But he is under pressure in Britain to stand up to Washington’s demands. He received heavy criticism for seeming to support Trump over Darroch — for failing, as the tabloids put it, to back up “our man in Washington.”

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The transfer of power in London will be quick.

On Wednesday, outgoing Prime Minister Theresa May will host her last question-and-answer session in the House of Commons and then travel to Buckingham Palace to formally resign. Johnson will follow her to the palace, where Queen Elizabeth II will ask him to form a new government. Johnson will be the 14th prime minister during the queen’s long reign.

The 55-year-old Johnson will take up residence at Downing Street and within hours begin announcing his cabinet. His 31-year-old girlfriend, Carrie Symonds, a former Conservative Party communications official and a top Tory spinner, may move in over the weekend, according to British news reports. Expect a lot of tabloid interest in this unprecedented arrangement.

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When Johnson clocks in, he will face an overflowing in-tray of items that need urgent attention, including a showdown in the Persian Gulf with a belligerent Iran. The two countries have been in a tense standoff since Britain impounded an Iranian tanker suspected of ferrying oil to Syria, and Iran retaliated by seizing a ­British-flagged oil tanker last week.

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Iranian Foreign Minister Mohammad Javad Zarif congratulated Johnson on Tuesday. “Iran does not seek confrontation,” Zarif tweeted. “But we have 1500 miles of Persian Gulf coastline. These are our waters & we will protect them.”

Political observers are keen to see whether Johnson continues Britain’s effort to salvage the 2015 deal designed to discourage Iran from developing nuclear weapons or bends to U.S. pressure to impose sanctions on Iran.

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But Johnson’s main challenge will be getting Britain out of the European Union.

May’s failure to deliver Brexit on time was the reason her Tory lawmakers ousted her.

Now Johnson, the face of the winning Brexit campaign in 2016, has vowed that, “do or die,” Britain will leave by the end of October. That gives him just three months to come up with a withdrawal plan that can win over both E.U. leaders and the British Parliament. He has threatened to depart without a deal, risking economic turmoil.

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Johnson joked with the Tory grandees, top donors and party activists at the announcement of his selection on Tuesday: “I read in my Financial Times this morning that there is no incoming leader, no incoming leader has ever faced such a set of daunting circumstances, it said. Well, I look at you this morning and I ask myself, ‘Do you look daunted? Do you feel daunted? I don’t think you look remotely daunted to me.’ ”

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Writing in the Telegraph, Johnson said this week that if the Americans could land men on the moon 50 years ago using hand-knit bits of computer code, 21st-century Britain could imagine a way to provide for frictionless trade across the border between the Irish Republic and Northern Ireland, which has been one of the stumbling blocks of the Brexit deal.

Tony Blair, a former Labour prime minister who opposes Brexit, was not impressed, telling the BBC that “the two things are obviously rather technically different.”

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The Europeans’ top Brexit negotiator, Michel Barnier, said his side looked forward to working “constructively” with Johnson. He hinted that the E.U. was prepared for some compromise, ready to “rework” the political declaration on future relations.

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In his remarks Tuesday, Johnson said Britain was at a historic crossroads. “We again have to reconcile two sets of instincts, two noble sets of instincts. Between the deep desire of friendship and free trade and mutual support in security and defense between Britain and our European partners,” Johnson said. “And the simultaneous desire, equally deep and heartfelt, for democratic self-government in this country.”

Afterward, Johnson met behind closed doors with Conservative lawmakers. According to a torrent of tweets from the meeting, he vowed to make Brexit a “towering success,” rebuild the navy and supply “broadband into every orifice of every home.”

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Nick Hargrave, a former special adviser to May, said the first days are “overwhelming” for all new governments. But in several tweets, he suggested that Johnson quickly make some key decisions: Does he want a no-deal Brexit? Or cosmetic changes to May’s withdrawal agreement? And is the pathway to get there a general election or a second referendum or a showdown with Brexiteers in his own party?

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Despite the do-or-die rhetoric, Johnson would prefer to leave with an amicable divorce, but not with May’s deal, which he called “dead.” 

All politicians fudge deadlines. But Steven Fielding, a political historian at the University of Nottingham, said, “Johnson has given himself no caveats with the 31st of October. That’s it.”

The majority of lawmakers in Parliament are opposed to a no-deal Brexit, signaling a potential clash. Some ministers are resigning their posts before Johnson can fire them over their resistance.

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On Monday, Alan Duncan quit his job as a Foreign Office minister. He complained that Johnson “flies by the seat of his pants, and is all a bit sort of haphazard and ramshackle.” And he told the BBC that a Johnson-led administration could go “smack into a crisis of government.”

Philip Hammond, the chancellor, and David Gauke, the justice secretary, also pledged to quit with Johnson’s ascension.

“Things are really about to kick off again in a massive way because the irresistible force of Boris Johnson’s ego is about to meet the immovable force of the House of Commons,” said Rob Ford, a politics professor at the University of Manchester.

Over the weekend, Simon Coveney, Ireland’s deputy prime minister, said the Irish government looks forward to engaging with the new British leader but warned against ripping up the existing agreement.

“If the approach of the new prime minister is they are going to tear up the withdrawal agreement, then I think we’re in trouble,” Coveney told the BBC. “That’s a little bit like saying, ‘Give me what I want or I’m going to burn the house down for everybody.’ ”

Michael Birnbaum in Brussels contributed to this report.