Some expect a masterclass in soft power. Others warn of a brutal humiliation. Few in the US will envy King Charles, a royal whose entire life has been governed by protocol, next week when he faces a man who glories in smashing it.

The British king will travel to Washington for a private tea and state dinner with Donald Trump at a moment when the “special relationship” has been plunged to its lowest point in 70 years by the US president’s war in Iran and belittlement of the prime minister, Keir Starmer.

Trump, for his part, may be hoping that the pageantry of a royal visit will provide a welcome distraction from domestic political troubles that have dragged his approval rating down to 33%. But his recent broadsides against the pope demonstrated his contempt for once sacred norms; Washington will be watching closely to see if Charles suffers a similar fate.

“The king is in a challenging spot,” said Brendan Boyle, a Democratic congressman from Pennsylvania. “He’s attempting to use the soft power of the position that he holds yet at the same time tiptoe around the delicate matters of Trump constantly bashing the British prime minister. It’s not exactly the sort of happy, festive ceremonial-type role that typically a visit like this would be.”

The four-day trip, the most high-profile of Charles’s reign so far, is ostensibly designed to mark the 250th anniversary of US independence from Britain. The 77-year-old monarch is sure to emphasise strong bonds between the two nations that transcend momentary spats. He arrives with cultural capital thanks to 79-year-old Trump’s reverence for the royals – the president is said to regard his visit to Windsor Castle last September as a highlight of his first year back in office.

Still, there could hardly be a more unlikely pairing than a staid monarch whose every move is exquisitely choreographed and a mercurial president seen by critics as an agent of chaos. The Save America Movement, a US pro-democracy group, argues that Charles is wrong to legitimise Trump’s extremism and is planning an aggressive advertising campaign to follow the king everywhere he goes.

Steve Schmidt, a veteran political strategist who sits on Save America’s steering committee, said: “He is coming at a moment where Trump is unhinged, waging an illegal war, plunging the world into chaos and yoking the western allies to a defeat by the world’s No 1 state sponsor of terror. While the king lives in rarefied air, he does live within the atmosphere of the Earth and the king’s decision here will be met with a fierce opposition.”

Schmidt even drew a comparison with Charles’s uncle, the Duke of Windsor (formerly King Edward VIII), meeting Adolf Hitler in 1937: “He will be standing next to a full-blooded American fascist who is on a constant basis threatening the peace of the world. The optics of it are awful. This is a moment where the crown is going to be used as a piece of propaganda.”

Images of Trump and convicted sex offender Epstein were projected on to Windsor Castle last year as the president arrived in Britain for a state visit. Some of the Save America Movement’s advertising will target the same issue.

Schmidt said Charles’s brother, Andrew Mountbatten-Windsor, who was arrested in February on suspicion of leaking government documents to the late US sex offender, “is a charter member of the Epstein class that Donald Trump is part of and the reality is everybody in the royal family knew exactly what Andrew was just as everyone around Trump knows exactly what Trump is so that’s going to be extremely uncomfortable. We’re going to do everything we can to raise that discomfort.”

The former Prince Andrew has denied any wrongdoing. Palace aides will be anxious to keep the king away from reporters who might shout questions about his brother’s ties to Epstein. Democrats in Congress are also pressing the issue. Last month Ro Khanna of California, co-author of the Epstein Files Transparency Act, wrote to Charles requesting a meeting with survivors of Epstein’s abuse but Buckingham Palace has ruled out such a meeting, citing legal advice.

On Tuesday Khanna will hold a public meeting on Capitol Hill with advocacy groups, the Epstein survivor Sharlene Rochard, and Sky and Amanda Roberts, members of the family of Virginia Giuffre, who alleged that she had been sexually trafficked to Andrew and who took her own life last year. Charles is due to address a joint meeting of Congress later that day.

Khanna told the Guardian: “The king, by meeting the survivors, would signal that, whether you’re a royal or a daughter of immigrants, you have human rights and that the modern monarchy stands for the human rights of every individual. Is it a vestige of the past that is a symbol of unaccountable power, of elite impunity, or is the monarchy going to be a constructive force in the 21st century standing up for human rights? The king has a defining moment.”

Charles will be the first British monarch to speak to Congress since his mother, Elizabeth II, shortly after the Gulf war in 1991. Although the speech is sure to be delicately crafted to avoid political controversy, Khanna added: “Even if the king does not visit with the Epstein survivors, I hope he will talk about the illegal, immoral war in Iran, the need for leadership on climate, the need to have allies and not just go it alone in the world. He hopefully will articulate a vision that isn’t just rubber-stamping Trump’s whims.”

Some in Britain bristle at Khanna’s intervention when there has been such scant legal or political reckoning over the Epstein files in the US. The British establishment, by contrast, has been shaken to the foundation with the arrest of Peter Mandelson, a former ambassador to the US, leaving Starmer fighting for his job.

Adding to the prime minister’s woes is a chasm that has opened between the White House and Downing Street over the Iran war. Trump singled out Starmer for failing to provide active military support for the US offensive, dismissing him as “no Winston Churchill” and deriding Britain’s aircraft carriers as mere “toys”.

Susan Page, the Washington bureau chief of the USA Today newspaper, said: “Between the war in Iran, the trouble with tariffs and the debate over the future of Nato, the special relationship is in as perilous a state as it’s been since it was formed.”

Page, author of a new book, The Queen and Her Presidents, notes that Charles’s mother was crucial in preserving the special relationship in times of trouble such as the Suez crisis in 1956. And he has at least made a positive start. “Charles has succeeded in cultivating a relationship with Trump,” she said.

“Their first meeting was not so great: Prince Charles met with Trump when he went to the United Kingdom during his first term and Trump came out afterwards and complained that he had only wanted to talk about climate change. But in their subsequent meetings, Charles has managed to develop a good relationship with Trump, and Trump speaks very warmly of Charles now.”

Indeed, asked by the BBC on Thursday whether the king’s visit could help repair the US-UK relationship, Trump replied: “Absolutely. He’s fantastic. He’s a fantastic man. Absolutely the answer is yes. I know him well, I’ve known him for years. He’s a brave man, and he’s a great man. They would absolutely be a positive.”

Both Charles, who visited the US 19 times as Prince of Wales, and Trump will speak at a state dinner in the White House East Room, which can seat about 200 people; it is rumoured in Washington that Melania Trump, the first lady, has intervened to trim the guest list and avoid overcrowding.

After navigating the political swamp of Washington, Charles and Queen Camilla will travel to New York to meet first responders and the families of victims of the September 11 terrorist attacks. The royal couple will then journey to Virginia to attend a street block party celebrating the 1776 Declaration of Independence.

Sean Spicer, who was Trump’s first press secretary in his first term, observed: “For a lot of Americans, we’re proud of our heritage and the relationship that we have between the two countries. It’s fascinating whether it’s the queen, the king, Diana – Americans have always had a affinity for the royals.”

But Sidney Blumenthal, a former aide to President Bill Clinton, has a more sceptical take. “Charles can come as an expert on the madness of kings,” he observed wryly. “He can read those relevant sections of the Declaration of Independence that are pertinent today. Presumably in his speech to the Congress, Charles should call for the full release of the Epstein files. The ghost of Jeffrey Epstein haunts everything.”