If Brexit — the most agonizing, paralyzing political crisis suffered by America’s closest ally in decades — is a gas can, President Trump may be a match.

Trump’s long-delayed state visit to Britain collides with the nadir, at least so far, of Britain’s long and tortuous withdrawal from the European Union. Because Trump is an eager cheerleader for Britain’s exit — with his own theories about how it should be done — it could complicate matters during the visit that begins Monday with lots of royal pomp and circumstance.

Add that Trump is a past critic of Brexit casualty Prime Minister Theresa May, scalped by her ruling Conservatives but still on the job for the duration of this trip. And he is chummy with Brexit frontmen Nigel Farage and Boris Johnson, the front-runner to be the next prime minister. Neither is on Trump’s public schedule, but they may turn up somewhere.

The expectation in Britain is that Trump won’t be able to help himself from lobbing a grenade or two into the country’s delicate political moment.

“He’ll say dreadful things about Brexit that will upset at least half the British population,” said Adam Thomson, a former British ambassador to NATO. 

But Thomson said Trump may find that any intervention has the opposite impact of what he hopes, because he is so deeply unpopular in Britain.

“An endorsement of Boris Johnson will hurt, not help, Boris,” said Thomson, who is now director of the London-based European Leadership Network. “Praise for a no-deal Brexit will hurt, not help, no-deal Brexiteers.”

In a pre-visit interview with London’s Sunday Times, Trump appeared to embrace the no-deal route. “If you don’t get the deal you want, if you don’t get a fair deal, then you walk away,” he said.

Trump embarrassed May during a less showy working visit in July by criticizing her approach to Brexit in a tabloid interview, saying she had failed to take his advice about how to execute the divorce and praising her rival Johnson. 

“This comes at such a difficult moment,” said Heather Conley, director of the Europe program at the Center for Strategic and International Studies. “We don’t know if there were lessons learned from their approach last year.”

Maybe, or maybe not.

Even before lifting off, Trump lauded Johnson and Farage.

“Nigel Farage is a friend of mine; Boris is a friend of mine,” Trump said Thursday, noting the strong performance of Farage’s ­Brexit Party in European Parliament elections. “Maybe it’s not my business to support people, but I have a lot of respect for both of those men.”

He followed that with an interview with the Sun tabloid, in which he did not offer a full endorsement but was quoted as saying: “I think Boris would do a very good job. I think he would be excellent. . . . I don’t know that he is going to be chosen, but I think he is a very good guy, a very talented person. He has been very positive about me and our country.”

Trump said that “other people have asked me for an endorsement,” while declining to specify who. A dozen Tories have announced bids for the top job.

Trump was also critical in the interview of May’s handling of the Brexit negotiations, saying that “the U.K. allowed the European Union to have all the cards. And it is very hard to play well when one side has all the advantage.”

Smart money is betting Farage and Johnson turn up at a Tuesday night dinner hosted by Trump pal Woody Johnson, the U.S. ambassador to Britain, at Winfield House, the stately parklike U.S. residence in central London.

Also on the trip agenda: Trump will spend time with Queen Elizabeth II and the royal family, observe commemorations of the 75th anniversary of D-Day in Portsmouth, England, and Normandy, France, and stay overnight at his golf course in Ireland.

May passed along the queen’s invitation for a state visit following Trump’s first week on the job, back in January 2017. But the threat of mass protests and the topsy-turvy nature of politics in Britain ever since have made it awkward to consummate. Repeated delays followed.

Now that it’s happening, it will lack many of the typical trappings.

Trump, for instance, will not stay at Buckingham Palace, which is under construction. He and first lady Melania Trump are not slated for a ceremonial carriage ride afforded other state guests — security concerns have been cited. And their formal welcome will not be at Horse Guards Parade, the usual venue, but in the palace’s private grounds.

Trump also won’t address Parliament. The U.S. side does not appear to have formally requested the privilege. Had it done so, House of Commons Speaker John Bercow indicated the answer would have been “no.”

Londoners opposed to Trump’s views on Europe, NATO, trade, human rights, climate change and other matters plan mass street demonstrations during the visit. The 20-foot Trump Baby balloon that miffed the president in July may fly again. Activists hoisted the diaper-clad (“nappy-clad” to Londoners) blimp for a test run but are waiting for one permit, and they also need to reach a fundraising target, to fly it above crowds that organizers said could reach 250,000 in London.

A senior U.S. official who briefed reporters about the trip downplayed any concern over protests or Trump’s unpopularity in Britain. 

Meanwhile, some British political figures have declined invitations to a Buckingham Palace banquet, citing differences with Trump. And American-born royal and former actress Meghan Markle, who called Trump “misogynistic” ahead of the 2016 election, is not expected to attend events with other members of the royal family. 

A British official would say only that Meghan, Duchess of Sussex, is on maternity leave and is not making public appearances. The official cited diplomatic custom in requesting anonymity to discuss plans for the visit.

In the Sun interview, Trump called Meghan “nasty.”

The Trumps will visit Westminster Abbey on Monday, and will lay a wreath at the Tomb of the Unknown Warrior ahead of the ­ruffles-and-flourishes banquet. 

Trump sees May on Tuesday; by Friday, she’ll be out as Conservative Party leader, though she’ll stay on as prime minister until a successor is chosen this summer. With her tenure ticking down, it is unlikely the talks will accomplish much. But she will be hoping for a clear endorsement from Trump of a future U.S.-Britain trade deal, which she could cite as an achievement amid the overall gloom of Brexit purgatory.

National security adviser John Bolton, who arrived in London days ahead of Trump, told Sky News on Thursday that the president “looks forward to a day the U.S. and the U.K. can negotiate a bilateral agreement that will be beneficial to both countries.” But Bolton signaled that even symbolic commitments may have to wait until there’s a new prime minister: “It’s going to be up to the Conservative Party . . . to pick a new leader and then to see what happens in the negotiation process with the European Union.”

Trump is expected to press May on at least a couple of fronts that the prime minister would prefer to avoid. He is likely to renew his push for Britain to keep Chinese telecommunications firm Huawei out of its 5G networks, something May’s government has resisted. And he has indicated he’ll raise his allegation that Britain worked with the Obama administration to “spy” on his 2016 campaign. British officials have dismissed the claim as baseless.

“It is hard to imagine anything, including a Trump visit, making British politics worse than they are now,” said Amanda Sloat, a former State Department official who specializes in Europe at the Brookings Institution. “He sees the European Union as an economic foe, welcomes Britain’s decision to leave the E.U. and has taken a predatory approach to bilateral trade talks.”

On Wednesday, Trump is scheduled to visit Portsmouth, the British port city where thousands of Allied troops embarked for the D-Day invasion in June 1944.

From there he’ll fly to Ireland — his first visit to the country since becoming president. A planned visit there in the fall was canceled amid protests. He’ll be staying clear of the crowds this trip, as he plans to stay at his private golf club. He’ll meet briefly with Irish Taoiseach Leo Varadkar at Shannon Airport — a compromise location, after the Irish reportedly balked at being Trump’s guests at his club.

On Thursday — the official anniversary of D-Day — Trump will take part in another commemoration, this one at the Normandy American Cemetery in France, where 9,380 U.S. military personnel are buried. He’ll meet with French President Emmanuel Macron and then return to his golf course.

The relationship between the two leaders has cooled, though Trump still talks admiringly of a Bastille Day parade he attended with Macron and has sought to bring something similar to Washington.

Trump received heavy criticism during his last trip to France, for a commemoration of the 100th anniversary of World War I, when he canceled a visit to an American cemetery due to rain.

A senior U.S. official who briefed reporters Thursday brushed off questions about whether it might be better to postpone Trump’s British visit until after British politics settles down a bit.

“There is no better time to have a visit to the United Kingdom than the 75th anniversary of D-Day,” the official said, adding later that “you can’t very well postpone D-Day” for the sake of political convenience.

 Witte reported from London. Karla Adam in London contributed to this report.