WASHINGTON — Since the start of the Trump administration, Britain has helped lead the quiet resistance to a president upending American foreign policy and straining the trans-Atlantic alliance.
That may change with Boris Johnson’s ascent to the post of prime minister of Britain on Wednesday. With his showmanship, his fondness for broad declarations and his transactional politics, Mr. Johnson, or “BoJo” as he is commonly known, is cut from Trumpian cloth.
“Britain is in an existential crisis, and the U.S. is in a form of crisis,” said R. Nicholas Burns, one of the top State Department officials under former President George W. Bush. “Both of their leaders are mercurial, and they’re entirely unpredictable.”
Mr. Johnson fueled the Brexit movement in 2016; Mr. Trump embraced it months later. That, in turn, helped Mr. Johnson raise his profile, including his two-year stint as Britain’s foreign minister under Theresa May, the outgoing prime minister, whose leadership he helped undercut.
Both men are forces looking to shatter decades-old institutions that have bound together Western democracies. Mr. Trump has talked of withdrawing from the North Atlantic Treaty Organization, while Mr. Johnson aims to carry out Brexit, Britain’s divisive plan to leave the European Union — even if he has to do it without an agreement with Brussels.
The big question in Washington is whether Mr. Johnson will further change Britain’s course and more closely align the nation with its American cousin.
When Mr. Trump pulled out of the 2015 Iran nuclear deal, the British joined France and Germany in designing a plan to preserve the accord, aimed at helping compensate Tehran and offset American sanctions.
When Mr. Trump attacked NATO, London reassured its allies about the centrality of the alliance, even if the United States was distancing itself.
And when the Americans demanded that the United Kingdom and other European nations ban next-generation telecommunications gear provided by Huawei, the Chinese company, the British government hesitated, saying that the threat might not be great enough to risk trade with Chinese firms.
Mr. Trump is clearly hoping that his British doppelgänger will share his instincts on these issues. As it became clear that Ms. May would have to step down because of the Brexit impasse, Mr. Trump emerged as Mr. Johnson’s strongest advocate among world leaders.
On Tuesday, Mr. Trump said in a speech that Mr. Johnson was “a really good man.”
“They’re saying ‘Britain Trump,’” Mr. Trump said. “They call him Britain Trump, and people are saying that’s a good thing. They like me over there. That’s what they wanted. That’s what they need.”
The two men also seemed to be on the same side in a diplomatic row two weeks ago after leaked cables written by the British ambassador to Washington, Sir Kim Darroch, described Mr. Trump as “clumsy and inept.”
Mr. Trump responded with a fusillade of tweets attacking the ambassador as “a very stupid guy” and Ms. May as “foolish,” and said the United States would no longer work with Mr. Darroch. The career diplomat resigned within days, but not before Mr. Johnson in a televised debate refused to defend Mr. Darroch or criticize Mr. Trump. (He later partially backtracked.)
That signaled what could be a wider shift in the dynamic between the United States and Britain as Mr. Johnson takes office.
If he appoints an ally to the top role in the United Kingdom’s embassy in Washington, it is hard to imagine the new envoy writing to Mr. Johnson’s government in the same frank language that Mr. Darroch had employed in his cables. Mr. Darroch, after all, had referred to Mr. Trump’s scrapping of the Iran accord as “an act of diplomatic vandalism, seemingly for ideological and personality reasons.”
Mr. Trump could ask Mr. Johnson to appoint Nigel Farage, the pro-Brexit right-wing politician, as Britain’s ambassador to Washington. On Tuesday morning, Mr. Farage was with Mr. Trump at an event in Washington.
Mr. Farage said Mr. Trump had told him that he and Mr. Johnson should push Brexit through the system.
“I want him to work with you, Nigel, to get the Brexit deal done,” Mr. Trump said, as recounted by Mr. Farage during an interview with a British radio show.
Another potential ambassador is Liam Fox, a pro-Brexit Scottish politician now serving as Britain’s secretary of state for trade and industry.
Mr. Trump is expected to continue dangling a potential free trade agreement with Britain to encourage Mr. Johnson to lead the nation in exiting the European Union by the deadline of Oct. 31. But given the complexities of British politics regarding Brexit, Mr. Johnson may find himself hard-pressed to meet Mr. Trump’s demands.
And given the all-consuming nature of figuring out Brexit, Mr. Johnson will have little time in the coming months to focus on other issues that might be central to the two nations.
“Nothing will come to pass anytime soon,” said Wendy R. Sherman, the third-ranking State Department official in the Obama administration and a key negotiator of the Iran deal. “Both Trump and Johnson may together set a new trend in hair styles, but that alone won’t restore a ‘special relationship.’”
Some European officials and analysts expect Mr. Johnson to act as more of a traditionalist than Mr. Trump. After all, he is a product of the British establishment — schooled at Eton and Oxford University, a member of Parliament, the former mayor of London. His stint as foreign minister from 2016 to 2018 resulted in some colorful quotes but no radical breaks from previous policies.
And for all his bluster and idiosyncrasies, Mr. Johnson does not rail against elite circles in the way that Mr. Trump does.
“People keep putting them in the same box,” said Leslie Vinjamuri, head of the United States and Americas program at Chatham House, a research group in London. “But I see them as very, very different kinds of people with quite different instincts and very different networks. Boris has been an insider all the way through — everybody knows him, everybody has worked with him.”
“Boris is highly intelligent,” she added. “He plays the game very differently.”
The test may come on Iran. Until now, the United States and Britain have been diametrically opposed over Mr. Trump’s “maximum pressure” campaign of harsh sanctions to force political change in Tehran.
Under Ms. May, and during Mr. Johnson’s tenure as foreign minister, Britain denounced Mr. Trump’s withdrawal from the 2015 nuclear agreement, insisting that it was working. With France and Germany, Britain sought to set up a barter system to get food, medicine and other goods to Iran, noting that they were not banned by the unilateral American sanctions.
European officials say Britain is still working in concert with the Europeans on Iran, despite the seizure last week of a British oil tanker by the Iranian military.
There is no sign yet that Mr. Johnson will abandon the European project to save the nuclear deal. And if Mr. Trump stumbles into a war with Iran, Mr. Johnson would almost certainly face great opposition from the British public if he tried to deploy its military to stand alongside the United States.
How Mr. Johnson will handle the fraught issue of China — on which Mr. Trump’s national security officials are in constant dialogue with their British counterparts — is also unclear.
Mr. Johnson and other British officials are fearful of wrecking London’s relationship with China, the world’s second-largest economy, and have refrained from using the same clash-of-ideologies language that Secretary of State Mike Pompeo and other Trump aides deploy when talking about their global rivalry with Beijing. And Mr. Johnson has made no proclamations yet about banning Huawei from Britain.
With the Brexit deadline looming, Mr. Johnson might find himself pushing off those decisions, which could create friction with the Trump administration.
“I’m not convinced it’ll go especially well,” Dr. Vinjamuri said. “They’ll appreciate each other’s willingness to play fast and loose with conventions and rules. But once you move to more serious issues, it’ll be hard to anticipate how that will go.”