Trump ditched diplomatic niceties that would be expected of a more conventional US leader on a pomp-laden day of royal ceremony after signaling that his vision for the "special relationship" requires Britain to fall into line with some of his hardline foreign policies.

Moments before landing outside the UK capital on Air Force One, he took an extraordinary shot at London Mayor Sadiq Khan, who over the weekend compared Trump's language to that of a 20th century fascist.

"@SadiqKhan, who by all accounts has done a terrible job as Mayor of London, has been foolishly 'nasty' to the visiting President of the United States, by far the most important ally of the United Kingdom. He is a stone cold loser who should focus on crime in London, not me," Trump tweeted.

"In any event, I look forward to being a great friend to the United Kingdom, and am looking very much forward to my visit. Landing now!"

Trump's trip comes at a moment when the United Kingdom is at risk of tearing itself apart amid its worst crisis since World War II over the divisive vote to leave the European Union.

UK Prime Minister Theresa May is the lamest of lame ducks, and will just cling on long enough until Friday to call a Conservative Party election to find her replacement in 10 Downing Street.

The prospect of fracturing ties with Europe means the royal pageantry Britain will lay on for Trump is about more than hospitality. The "special relationship" is increasingly vital for the UK government.

The hope is that Trump, whose addiction to adulation means he's susceptible to pageantry, will favor the land of his Scottish mother's birth -- when Britain seeks a free trade deal with the US if it finally works out how to leave the EU.

Trump may also be looking to forge a closer bond with the next prime minister than the cordial, yet occasionally awkward relationship he maintained with May.

"President Trump is very much looking beyond May, and I think his message is very much targeted at the next British government," said Nile Gardiner, director of the Margaret Thatcher Center for Freedom at the Heritage Foundation, a conservative Washington think tank.

"President Trump sees Mrs. May's handling of Brexit as disastrous."

May, however, said in a welcoming statement that she looked forward to building on ties between the two countries.

"This is a significant week for the special relationship and an opportunity to further strengthen our already close partnership," May said.

"Our relationship has underpinned our countries' security and prosperity for many years -- and will continue to do so for generations to come," she said.

British officials say that Trump's team has gone out of its way to cooperate with them over the arrangements for his visit.

A UK government official insisted that the talks between May and Trump, who have met often and speak frequently by phone, will not be "remotely awkward."

"It'll be fine. And I can promise you the talks will be important and substantive," the official said.

The trip will give Trump the chance to bask in the heavily staged theatrics of British ceremony, to frame himself as a statesman and to position his family -- his adult children will be at his side -- as an American political dynasty.

Still, Trump often has a tendency to trample over the carefully choreographed imagery that surrounds presidential travel, by igniting political conflagrations often set off by his erratic reactions to events back home.

And Trump will not get all the trappings that his predecessors sometimes enjoyed. He was not invited to address Britain's parliament, for instance, amid opposition to his travel ban on residents of some Muslim nations that was criticized by John Bercow, the speaker of the House of Commons.

Turmoil follows Trump wherever he goes

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Trump arrives in London amid a growing political showdown with Democrats over their attempts to investigate his campaign and his presidency. Trump's trip also comes during an escalating trade war with China and after the President threatened to impose new tariffs on Mexico in a bid to stop the flow of migrants toward the US border.

The political tumult gave Trump an opening for mischief before he even left the US on a trip that will also see him travel to France to honor the 75th anniversary of the D-Day Normandy landings and will also include a stop at his golf resort in Ireland.

He's already backed Tory leadership contenders, such as Boris Johnson, who are adamant about honoring the populist vote in 2016 to leave Europe that foreshadowed Trump's own election.

The administration has been sending strong hints that for the special relationship to come to fruition in the Trump era, Britain should more closely hew to the US line on key world issues such as Iran and China and ought to leave the EU.

Given the UK's political uncertainty, and Trump's unpopularity here, which has made the long-delayed visit a political hot potato for May, one key question is why is the state visit happening at all right now.

"We felt, all of us, that this is extremely important," a senior White House official told reporters last week.

"The President has said this over and over again about the unshakable bond between the two countries. "But even in the most difficult times where you may have political upheaval and uncertainty ... we need to stand together, shoulder to shoulder."

'Get the deal closed'

Before traveling to the UK, Trump leaned heavily on the scales of the Conservative leadership election, calling for the next Prime Minister to finally follow through on the 2016 referendum.

"They've got to get it done," Trump told the Sunday Times in an interview. "They have got to get the deal closed."

The President also praised Nigel Farage , whose new Brexit party emerged victorious in recent European elections and is now posing an existential threat to the Conservative Party.

Britain was forced to extend its membership of the EU until the end of October after May failed to get a separation agreement with Brussels agreed by Parliament after several votes.

It now faces the possibility of dropping out of the bloc without a deal -- a scenario that could cause severe economic disruption -- or possibly having a rethink about the decision to leave.

Trump and his national security adviser, John Bolton, have made no secret of their desire to see Britain sever ties with the EU -- the kind of multilateral arrangement abhorred in their worldview rooted in national sovereignty.

The administration may also hope to peel the UK away from European regulatory standards in areas such as food and agriculture and is likely to drive a hard bargain in trade talks, despite the warm words about the possibility of such a deal. There's also uncertainty over whether any trade deal could get ratified by Congress given the looming 2020 US election.

Bolton, who was in London ahead of the Trump visit, has close connections in British conservative circles and appears to be rooting for an ideological soulmate for the President in London.

But Trump's intervention in the Tory leadership race comes with risks. It is likely to further antagonize his British critics and fuel protests that are expected in London during his visit.

Trump's defenders note that his predecessor President Barack Obama made a forcible intervention in the Brexit debate, warning before the referendum that Britain would go to "the back of the queue" for a trade deal with the US if it left Europe.

Trump's decision to play politics in Britain has already deepened the rift with the Labour opposition of Jeremy Corbyn -- a vehement Trump critic who refused to show up at a state banquet for the President hosted by Queen Elizabeth II.

"President Trump's attempt to decide who'll be Britain's next PM is an entirely unacceptable interference in our democracy," Corbyn tweeted over the weekend.

The Labour leader is hoping to make the next Conservative prime minister's tenure short lived by forcing a general election. Should Corbyn win, he would be the most left-wing and potentially the most anti-American British prime minister ever in a twist that could make current US-UK sensitivities pale by comparison.

'Nasty'

The President also risked creating a few awkward moments with the Royal Family ahead of Monday's sumptuous banquet at Buckingham Palace.

He said in interviews with two British newspapers that he didn't realize that the Duchess of Sussex, the American-born former actress Meghan Markle, had been "nasty" when he was told she once called him misogynistic.

But as a storm mounted over the remark, he tweeted on Sunday morning, "I never called Meghan Markle 'nasty.'"

The Duchess is not expected to attend Monday's state banquet as she is on maternity leave after the birth of her first child with Prince Harry. But Trump and first lady Melania Trump are due to have tea with Markle's new father-in-law Prince Charles.

Possible political flash points during the trip could come over what UK officials privately say is heavy US pressure for Britain to bar China's Huawei from building part of the country's new 5G network. The administration, which fears China could embed the new technology with surveillance capabilities, has warned the move could hamper intelligence sharing between the two "Five Eyes" allies.

Britain also remains committed to the Iran nuclear deal from which Trump withdrew the United States ahead of a campaign of intense economic pressure directed at Tehran's clerical rulers.

And his son-in-law Jared Kushner's forthcoming Middle East peace plan appears to conflict with the UK's position that a two-state solution is the way to end the Israeli-Palestinian conflict.