LONDON—Boris Johnson handily won the race to lead the Conservative Party on Tuesday, and will be Britain’s next prime minister within a day.

Johnson — a bombastic, Latin-quoting, Oxford classicist with a mop of intentionally mussed yellow hair — made his name as an over-the-top journalist in Brussels and then as London mayor and galvanized the successful Brexit campaign in 2016. He will walk through the black enamelled door of 10 Downing St. on Wednesday — fulfilling what his biographers describe as his relentless “blond ambition” to follow his hero, Winston Churchill, into the top spot.

In a leadership contest involving only dues-paying members of the Conservative Party, the former foreign secretary Johnson faced the current foreign secretary, Jeremy Hunt.

Johnson captured 92,153 votes to Hunt’s 46,656 — a dominant victory that shows Tories want a leader who promises, above all else, to deliver Brexit.

U.S. President Donald Trump, who hasn’t been shy about his admiration for Johnson, tweeted: “Congratulations to Boris Johnson on becoming the new Prime Minister of the United Kingdom. He will be great!”

Iran’s foreign minister, Mohammad Javad Zarif, also congratulated Johnson — though the two countries have been in a tense standoff since Iran seized a British-flagged oil tanker last week in the Strait of Hormuz. “Iran does not seek confrontation. But we have 1,500 miles of Persian Gulf coastline. These are our waters and we will protect them,” Zarif said.

Now, the transfer of power in London will happen quickly.

On Wednesday, outgoing Prime Minister Theresa May will deliver her last remarks at a question-and-answer session in the House of Commons and then she will travel to Buckingham Palace to resign. Johnson will follow her to the palace, where Queen Elizabeth II will ask him to form a new government. Johnson will be 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 new 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 press reports. Expect a lot of tabloid press interest in this unprecedented arrangement.

As a sign how the Conservative Party has torn itself apart over Brexit, the Tory backbencher Charles Walker asked the audience in the hall where Johnson’s victory was announced, “can we be kinder to the next prime minister than we have been to the current prime minister?”

The Conservatives hope that a Johnson-led administration will help to fend off Nigel Farage’s Brexit Party, which topped the polls in the recent European elections.

In his brief remarks after the win, Johnson praised Hunt, calling him a “formidable” campaigner who was friendly, good-natured and a “font of excellent ideas, all of which I propose to steal forthwith.”

Johnson joked with the audience of Tory grandees, top donors and party activists in attendance, “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.”

Signalling that the clock is already ticking, the arch-Brexiteer Farage reminded Johnson of his pledge to get Britain out of EU by Halloween. “Does he have the courage to deliver?” Farage asked.

The opposition Labour leader Jeremy Corbyn didn’t let the hour pass before firing his first salvo, saying, “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. But he hasn’t won the support of our country.”

Corbyn said that a no-deal Brexit “would mean job cuts, higher prices in the shops, and risk our NHS being sold off to U.S. corporations in a sweetheart deal with Donald Trump. The people of our country should decide who becomes the Prime Minister in a General Election.”

When Johnson clocks in for his first day of work in the top job, he will face an overflowing in-tray of daunting problems that need urgent attention, including — but not limited to — a showdown in the Persian Gulf with a belligerent Iran, vexing Brexit, assembling a top leadership team, the survival of his Conservative Party, ministerial resignations, rebels in Parliament and a raft of domestic issues ranging from housing to health care.

And Trump. The postwar “special relationship” has had a rocky month, as the American president lashed out on twitter against the British ambassador in Washington, calling him “a pompous fool.”

Sir Kim Darroch provoked the president’s ire when a cache of secret diplomatic cables were leaked to a British tabloid. The memos from Darroch described Trump as “insecure” and his administration as “inept” and “dysfunctional.” Darroch resigned in the aftermath — after Johnson failed to back up, as the tabloids put it, “our man in Washington.”

Also looming are new redlines and deadlines in the mess called Brexit. May’s failure to deliver Brexit on time was the reason her Tory lawmakers ousted her.

Johnson, who was the face of the winning Brexit campaign in the June 2016 referendum, has vowed, “do or die,” Britain will leave the European Union in October.

Writing in Monday’s Telegraph, Johnson said, “it is time this country recovered some its can-do spirit.” He said that if the Americans could land men on the moon 50 years ago using hand-knit bits of computer code, then 21st century Britain could imagine a way to provide for frictionless trade across the Northern Irish border, which has been one of the stumbling blocks of the Brexit deal.

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

The European’s top Brexit negotiator, Michel Barnier, said his side looked forward to working “constructively” with Johnson to help him ratify the existing withdrawal agreement that May negotiated — and that the Europeans have said they will not reopen.

Barnier said the EU was prepared for some compromise, ready to “rework” the declaration on future relations.

In his remarks Tuesday, Johnson said Britain was at a crossroads in his relations with Europe. “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 defence between Britain and our European partners,” Johnson said. “And the simultaneous desire, equally deep and heartfelt, for democratic self-government in this country.”

Johnson noted that many say that the two desires cannot be reconciled.

Facing Johnson is the same math in the House of Commons that defeated May’s Brexit deal three times. The incoming prime minister will have a paper-thin working majority, protected by the Democratic Unionist Party in Northern Ireland.

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As Brexit churns on, the Persian Gulf crisis threatens. The Iranians seized the British-flagged vessel (with an international crew, no Britons aboard) after Britain took an Iranian tanker in the Gibraltar Strait that London said was heading toward Syria.

Johnson doesn’t have the best track record of diplomacy with Iran. When he was foreign secretary, Johnson mistakenly said that Nazanin Zaghari-Ratcliffe, a British-Iranian woman visiting family, was teaching journalism in Iran. Zaghari-Ratcliffe was jailed for alleged espionage and her family said that Johnson’s comments didn’t help her case.

The new leader will in short order also choose a top leadership team, likely rewarding those who supported him and disappointing those who don’t get tapped for top jobs.

Johnson has warned that he will require that those who serve be prepared, as he is, to leave the European Union with no deal — a prospect that frightens many economists and leaders of British businesses, fishing and agriculture, who rely on tariff-free trade with the continent for their profits.

After a chaotic spring that saw Britain blow past its March 29 deadline to leave the EU, things seem to have calmed down. But not for long.

After the new leader is installed in 10 Downing St., he will have just three months to come up with a plan that can win over both EU leaders and the British Parliament.

Nick Hargrave, a former special adviser at 10 Downing St., argued that the first two days are “overwhelming” for all new governments. But in a series of tweets, he suggested that Johnson quickly make a few 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?

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

Unlike his rival Hunt, Johnson didn’t give himself wiggle room on the deadline.

“Most politicians say one thing but they are actually saying something else, it’s not definite as you might think,” said Steven Fielding, a political historian at the University of Nottingham.

But in Johnson’s case, he said, “he has given himself no caveats with the 31st of October. That’s it.”

British parliamentarians have been laying down a marker in hopes of preventing a no-deal Brexit, but it’s unclear how effective they could be.

The majority of lawmakers in Parliament are opposed to a no-deal Brexit, signalling a potential showdown to come. Some ministers are resigning their posts before Johnson can fire them over their opposition to his willingness to leave the bloc without a divorce deal.

On Monday, Alan Duncan quit his job as a foreign office minister. He said that Johnson “flies by the seat of his pants, and is all a bit sort of haphazard and ramshackle.”

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 their posts if Johnson becomes prime minister.

“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 that the Irish government looks forward to engaging with the new British leader but warned against ripping up the existing agreement.

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