President Trump Donald John TrumpBiden on Trump's refusal to commit to peaceful transfer of power: 'What country are we in?' Romney: 'Unthinkable and unacceptable' to not commit to peaceful transition of power Two Louisville police officers shot amid Breonna Taylor grand jury protests MORE dived headfirst into Great Britain’s fraught domestic politics on Tuesday, upending diplomatic norms at a pivotal point in the “special relationship” between the U.S. and U.K.

From escalating his feud with London’s mayor to weighing in on Brexit and the race to replace British Prime Minister Theresa May Theresa Mary MayAre US-Japan relations on the rocks? Trump insulted UK's May, called Germany's Merkel 'stupid' in calls: report Bolton says Boris Johnson is 'playing Trump like a fiddle' MORE — even speaking with one of her possible successors — it appeared there was no topic Trump wanted to avoid on British soil.

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Trump’s penchant for injecting himself into other countries’ internal matters makes life difficult for his hosts and, in the case of U.S. allies, puts a strain on key relationships, experts say.

“In general, it’s wise for foreign leaders to stay out of domestic politics because you’re just going to irritate a large number of people who disagree with whatever you say, and you probably won’t move the needle in the argument,” said Sebastian Mallaby, senior fellow at the Council on Foreign Relations.

Supporters of the president say he is a counterpuncher who is not afraid to break norms in order to hit back at critics, foreign or domestic, and shape world events.

That rule for Trump does not apply just to the U.K.; he has also spoken about the yellow vest protests against French President Emmanuel Macron Emmanuel Jean-Michel MacronNavalny released from hospital after suspected poisoning US-China tensions shadow United Nations meeting The US is missing an opportunity in Lebanon MORE, the next leader he will meet on his European trip.

“People ask me questions,” Trump told a reporter before departing the U.S. who asked if it is appropriate for him to comment on Brexit and the U.K.’s upcoming elections. “Don't ask me the question if you don’t want me to talk about it.”

Trump’s predecessors typically followed a different standard during their overseas visits, choosing to speak broadly or not at all when asked about their allies’ domestic affairs.

“This is not the way that American presidents act overseas,” Nicholas Burns, a top State Department official under former President George W. Bush, said on MSNBC. “Frankly, it’s embarrassing to see the American president call out people in this savage, brutal political tirade of criticizing opposition leaders in another allied country. It just makes life difficult for his hosts.”

There have been notable exceptions, however.

President Obama during an April 2016 visit to London put himself directly in the debate over Brexit by saying that Great Britain would be at the “back of the queue” in getting a trade deal with the United States if it voted to leave the European Union.

Obama also wrote an op-ed in The Telegraph calling for voters to remain in the EU, a position for which he was criticized by conservative members of Parliament.

Trump’s commentary could have an effect on upcoming Conservative Party elections, the winner of which may become his negotiating partner in a possible U.S.-U.K. trade agreement and his potential foil in his long-running feud with the North Atlantic Treaty Organization and other international institutions.

While Trump set aside his past criticism of May at their joint press conference on Tuesday, he didn’t shy away from offering his opinion on whether the U.K. should go ahead with plans to leave the EU or if he could eventually work out a trade deal with Labour Party leader Jeremy Corbyn if the Trump critic eventually becomes prime minister.

“I would think that it will happen, and it probably should happen,” Trump said at Tuesday’s press conference when asked about Brexit, standing beside May in front of a bank of U.S. and U.K. flags.

On the prospect of working with Corbyn, Trump said he turned down the chance to meet with him in part because he is “somewhat of a negative force.”

“I think that the people should look to do things correctly as opposed to criticize. I really don’t like critics as much as I like and respect people that get things done,” Trump said.

Yet criticize is a large part of what Trump did before and during his visit to the U.K.

The president had even harsher comments for London Mayor Sadiq Khan, who said Trump did not deserve the red-carpet treatment during his three-day state visit and likened him to a fascist. Trump responded by calling him a “stone cold loser” just before Air Force One touched down Monday at Stansted Airport.

“I think he’s been a not very good mayor, from what I understand,” Trump said when asked about Khan. “I don't think he should be criticizing a representative of the United States that can do so much good for the United Kingdom.”

Trump also waved off thousands of demonstrators who took to the streets to show their opposition to him, calling the protests “fake news” and saying he experienced only “great love” during his time there, which was almost exclusively spent in cordoned-off areas of central London.

As much as Trump rocked the boat, some watchers of the U.S.-U.K. relationship predicted the two countries would continue to work in harmony.

They pointed to the conciliatory tone Trump used toward May and his promise that Washington would not limit intelligence sharing with London amid a dispute over the Chinese telecom giant Huawei.

“[Trump’s] criticisms of the U.K. sort of come and go, and I think at some level even this president knows it’s an incredibly important partnership,” said Leslie Vinjamuri, head of the U.S. and the Americas program at the London-based think tank Chatham House.

Yet Vinjamuri conceded it was “uncharacteristic” and “unusual” for Trump to comment specifically on who should be May’s successor.

In an interview last week with the British tabloid The Sun, Trump said Conservative Party politician Boris Johnson would make an “excellent” prime minister. The two men spoke over the telephone on Tuesday morning, and Trump reiterated during his press conference that he’s a fan of the former London mayor and foreign secretary.

“I think he'd do a very good job,” Trump said, offering similar praise for current U.K. Foreign Secretary Jeremy Hunt, another leadership candidate who was seated in the front row. Trump jokingly asked Hunt if his other rival, Michael Gove, would “do a good job” and then turned and smiled at a laughing May.

“Donald Trump may have just picked out the next Tory prime minister,” read one headline in the online edition of The Telegraph newspaper.

A Trump endorsement, however, might be counterproductive for U.K. leadership candidates given that he has a 21 percent approval rating there, according to a YouGov survey.

Corbyn, meanwhile, seemed to relish being in Trump’s crosshairs. He told a large group of protesters in Trafalgar Square that the "far right" in the U.K., Europe and the U.S. has "no answers."

That dynamic did not deter Trump, who returned to the U.S. ambassador’s residence at the Winfield House early Tuesday evening immediately after the press conference to meet with Brexit Party leader Nigel Farage.

While Farage is not in contention to replace May, he has been a fierce advocate of the movement to leave the European Union and a vocal Trump supporter. The two men dined at Trump’s Washington, D.C., hotel about a month after the president’s inauguration.

“He is very interested as to who the next Conservative leader and prime minister is,” Farage told British radio station LBC following the meeting.

Vinjamuri said those seeking to replace May are more focused on strengthening support among fellow party members than they are about currying favor with Trump but noted that whoever serves as the next prime minister will inevitably have to forge a relationship with the president.

“Of course, if they’re successful, having the ability to work effectively with the president of the United States is critical,” she said. “So they are taking him seriously for that reason.”