LONDON — Thirty-five police cadets stood at attention, a perfect made-for-TV backdrop, as Prime Minister Boris Johnson strayed from a police recruitment announcement to trash his political opponents for opposing his Brexit plans and for plotting to keep Britain “incarcerated” in the European Union.

American politicians have long used soldiers and police as props during political speeches. But in Britain, politicians have avoided such staging as inappropriate and tacky — until the arrival of Johnson, the country’s convention-buster in chief.

“Boris is tearing up the rule book,” said Mujtaba Rahman, an analyst with Eurasia Group.

His tactics have repeatedly defied tradition and precedent in a society that treasures yesterday as a guide for tomorrow.

Britain famously has no written constitution. Its fundamental guide is not a document but an ethos — a centuries-long history of laws, legal rulings and traditions combined with a collective national assumption about the way things should be done.

In his six weeks in office, Johnson has detonated many of those assumptions in his effort to keep his promise to lead Britain out of the European Union by Oct. 31.

He ordered Parliament closed for five weeks. Suspensions themselves are not unusual but typically are not for that long, and not in the middle of pivotal moments in history.

He ejected 21 lawmakers from his Conservative Party for voting against him. They included some of the party’s most iconic figures and even Winston Churchill’s grandson, Nicholas Soames.

He said “shit” on the floor of Parliament, on national television.

He called his chief rival, Labour Party leader Jeremy Corbyn, a “chlorinated chicken” during a Parliament debate, instead of the traditional “right honorable gentleman.” Even more inexplicably, he then hurled another schoolboy taunt at Corbyn, calling him a “great big girl’s blouse.”

After all that, Johnson has refused to say whether he would comply with a law passed by Parliament to avert an abrupt and chaotic “no-deal Brexit” next month, without economic and social safeguards agreed with the E.U. “I would rather die in a ditch” than request another Brexit delay, he said — setting up an unprecedented standoff.

A cross-party group of lawmakers have lined up a legal team and are prepared to go to court to force Johnson to comply with the law, the BBC reported Saturday.

Alexander Boris de Pfeffel Johnson, 55, has the most establishment background possible: educated at the elite Eton and Oxford. But he has never been conventional or stuffy, and even his scarecrow thatch of blonde hair refuses to conform.

As a foreign correspondent in Brussels, he made a name for himself with incendiary, and factually dubious, stories about the bumbling and planning of E.U. bureaucrats.

As mayor of London from 2008 to 2016, he was perhaps best known for getting stuck dangling from a zip line during the Summer Olympics.

And when he resigned as Britain’s foreign secretary last year, he undiplomatically called then-Prime Minister Theresa May’s Brexit plan a piece of excrement.

Johnson has been considered a shrewd politician, who long aspired to Britain’s top job. But, as some of his un­or­tho­dox tactics begin to backfire, he may not be able to keep the job for long.

He does not have a single legislative win in his six weeks at 10 Downing Street. Livid lawmakers, in what little time they had left before the scheduled suspension, delivered him three embarrassing defeats last week: taking control of the legislative agenda, disrupting his Oct. 31 Brexit plan and rejecting his call for a general election on Oct. 15.

Johnson is expected to lose a second call for an election Monday. His opponents say they worry he’d manipulate the calendar to force through a no-deal Brexit. Opposition parties have found new cohesion in standing firm against his demands.

Meanwhile, an open rebellion in Johnson’s Conservative Party has cost him his governing majority, making it practically impossible for him to move even non-Brexit legislation. Conservatives, who were already divided about whether to accept a potentially destabilizing no-deal Brexit, are more deeply split than ever.

Johnson paid a deep personal price for his blunt-instrument tactics on Thursday, when his younger brother Jo Johnson resigned as a Conservative legislator and a government minister, citing “unresolvable tension” “between family loyalty and the national interest.”

The prime minister’s nightmare week ended with his speech before the West Yorkshire police cadets, where he stumbled and bumbled for a full minute, trying unsuccessfully to recite the police caution — Britain’s equivalent of the Miranda rights.

As he spoke, a cadet standing behind him fainted.

Mark Burns-Williamson, the West Yorkshire police commissioner, called Johnson’s performance a “political stunt” and demanded an apology. He said police had been told Johnson planned to focus solely on public safety spending.

“It clearly turned into a rant about Brexit, the opposition and a potential general election,” Burns-Williamson told reporters. “There’s no way that police officers should’ve formed the backdrop to a speech of that nature.”

Johnson is part of a wave of populist leaders around the globe who are challenging norms and asking why things can’t be done differently. Leaders in Hungary, Poland, Italy, the Philippines and elsewhere are doing and saying things that seemed unthinkable just a few years ago.

The leader of the disrupters is, of course, President Trump, who has shattered every possible kind of presidential norm. While Johnson and Trump are vastly different in many ways, they seem to feed off each other and admire each other’s nonconformist styles.

“Boris knows how to win. Don’t worry about him,” Trump said last week.

“He has many, many good qualities,” Johnson said about Trump this summer.

Jon Tonge, a politics professor at the University of Liverpool, said Johnson’s speech in front of the cadets was “straight from the Donald Trump playbook, deliberately imported . . . the Trumpization of U.K. politics.”

Emily O’Reilly, the European Union’s ombudsman, said Johnson and Trump have both succeeded through force of personality and an “intuitive sense of what appeals to their base, often to the darker or more anarchic, unconventional side of that base.”

She noted that in the United States and Britain there has been a “significant exodus of politicians from the center to what had previously been seen as the extreme.”

O’Reilly said the events of last week “suggest that Johnson may have gone too far in . . . the smashing of parliamentary and party conventions.”

Several legal challenges have been filed against Johnson’s moves. A high court judge in London last week sided with Johnson, ruling that he not acted improperly by ordering Parliament shut for five weeks.

“He’s a rule bender rather than a rule breaker,” Tonge said.

Britain’s lack of a written constitution leaves the country’s politics particularly vulnerable to someone like Johnson, who may follow the law but chooses not to follow established norms.

“We haven’t previously in the U.K. faced a leadership that was prepared to ignore conventions and precedents,” said Meg Russell, director of the Constitution Unit at University College London.

“It’s putting a great deal of pressure on our traditional constitution,” Russell said. “It could ultimately lead to pressure to nail down these rules more firmly.”

Rob Ford, a political analyst at the University of Manchester, said Britain has no specific laws governing how to interpret and implement referendums. He called that a “time bomb.” The 2016 Brexit vote was close, and the political class can’t agree on how to implement its results, he said.

Johnson is bending every rule he can think of to push his interpretation — that the will of the people was to exit the E.U. no matter the terms. His opponents say voters didn’t agree to a no-deal Brexit with potentially damaging repercussions, including predicted shortages of food and medicine.

Ford said the “only referee” in that argument is the public, so another vote seemed likely — either the general election Johnson wants, or the second referendum some anti-Brexit opponents favor.

Many politics watchers blame Johnson’s unconventional Brexit tactics on his chief strategist, Dominic Cummings, who masterminded the successful pro-Brexit campaign in 2016. Some here compare Cummings to Stephen K. Bannon, the controversial former chief strategist to Trump.

Former Conservative prime minister John Major, in remarks widely interpreted to be a swipe at Cummings, warned in a speech in Glasgow on Thursday against “political anarchists” who threatened to “poison the political atmosphere beyond repair.”

Even if Johnson crashes out of office soon, he has exposed complaints with the way the system was working for many people.

“There were legitimate grievances; the established class failed to a certain extent,” which gave rise to unconventional leaders in the United States and Britain, said Benjamin Haddad, director of the Atlantic Council’s Future Europe Initiative. “If you feel you have been left behind, that the system is not working for you, a more brash, direct rhetoric appeals to many people.”

Haddad said the challenge is to address what was not working: “We need to reinvent the way we conduct politics, the way we find answers to people’s questions.”

Johnson’s behavior has brought thousands of people into the streets across Britain in recent days. People who support Brexit and those who oppose it joined forces to protest Johnson’s norm-busting.

Nigel Nott, 70, a retired civil engineer protesting in London, said Johnson was breaking tradition to achieve his goals, and “to hell with everyone else.”

“The unwritten constitution works when you have respect and trust in people,” he said. “It does not work in the present circumstances.”

Mary Jordan in Washington contributed to this report.