It’s good to be the king. But it’s not without its traps, as King Charles III learned last weekend when the organizers of his coronation invited millions of Britons to pledge an oath of homage to the monarch during the ceremony on Saturday.

“A spectacular misjudgment,” said Graham Smith, whose group, Republic, wants to abolish the monarchy. “Discordant and potentially tone deaf,” posted Anna Whitelock, an expert on the monarchy at City, University of London. “More like the stuff of a Stalinist people’s republic,” wrote the columnist Mick Hume.

The archbishop of Canterbury, the Most. Rev. Justin Welby, who will preside over the service, insisted that the oath would be purely voluntary. It was meant as a democratizing gesture: At Queen Elizabeth II’s coronation in 1953, only members of the hereditary aristocracy swore allegiance.

Such are the problems vexing Charles as he prepares for his coronation, Britain’s first in 70 years. In the seven months since he ascended the throne, royal watchers say, the new king has worked to make the monarchy more accessible, forward looking and inclusive. Yet the hoary rituals of the coronation are a reminder of how — in a secular, multiethnic, digital-age society — the crown is fundamentally an anachronism.

As Buckingham Palace dusts off its royal relics — gleaming swords and scepters, a bejeweled orb and gold stage coach — Charles, 74, is walking a tightrope between tradition and modernity. People who know him say he knows he must adapt the institution to a society that has not necessarily turned against the idea of a king, but finds the trappings of royalty increasingly irrelevant.

His partner in this project is his 40-year-old son and heir, Prince William. The two have grown close after the painful rift between them and Charles’s younger son, Prince Harry, according to those who know them. They form the nucleus of a shrunken royal family, one that its defenders say will make fewer demands on Britain’s public purse.

“Under Charles and William, they’re going to work even harder to be relevant,” said Paddy Harverson, who served as communications secretary to Charles from 2003 to 2014. “He will want to manage spending more carefully and produce a more cost-efficient monarchy. He’s got a license to change things, but it will be gradual.”

Critics warn that public attitudes are changing faster than the monarchy. In a recent poll by the market research firm YouGov, 58 percent of people said Britain should continue to have a king, while 26 percent said it should have an elected head of state. But among those aged 18 to 24, fewer than a third favored keeping it.

“They are completely underestimating the change in the public mood,” said Catherine Mayer, who wrote a 2015 biography, “Charles: The Heart of a King.” “What we are witnessing is not the end of the monarchy,” she continued. “What we are witnessing is the end of the popular monarchy.”

Part of the problem is Charles himself. He has evolved, in a relentlessly documented life, from a gawky youth to a self-assured elder statesman. But he remains, in Ms. Mayer’s words, a “Marmite figure,” either loved or hated — much like the salty brown English spread that is marketed as the ultimate acquired taste.

That sets him apart from Queen Elizabeth, who was revered as a unifying figure, serenely operating above politics, a timeless counterweight to the daily upheavals of Britain’s parliamentary democracy.

In his brief reign, Charles has already found himself drawn messily into politics. In February, he welcomed the president of the European Commission, Ursula von der Leyen, to Windsor Castle, just hours after she signed an agreement with Prime Minister Rishi Sunak to settle a trade dispute in Northern Ireland.

Opponents of the deal said the king had allowed himself to be exploited by the government. Downing Street labeled it the Windsor Framework, which some said improperly put the king’s imprimatur on the agreement, since Windsor is not only one of the king’s homes but his family name.

Last fall, days after he ascended the throne, Charles reluctantly yielded to advice from Mr. Sunak’s predecessor, Liz Truss, not to attend the United Nations’ climate change conference in Egypt, despite his longstanding commitment to climate and environmental issues.

These episodes capture the challenge Charles will face as he adjusts to the nonpolitical life of a monarch: He is fervently committed to causes from organic farming to traditional architecture. He reads voraciously, aides say, and approaches public debates with the instincts of a contrarian. A former aide, Mark Bolland, once described him as a “dissident working against the prevailing political consensus.”

Experts on the monarchy predicted Charles would find other ways to channel his activism. Some predicted he would promote fine arts — classical music and the works of Shakespeare are particular passions, they said — more than the queen, whose outside interests ran mainly to breeding racehorses.

“He takes a view even more strongly than the queen that the monarchy has to be shown to be useful,” said Vernon Bogdanor, an authority on the constitutional monarchy. “He will use his soft power to a great extent.”

Charles’s diplomatic outreach got off to an awkward start when the king played host at a state banquet for President Cyril Ramaphosa of South Africa, one of the Commonwealth nations. A week later, Mr. Ramaphosa faced an impeachment vote on charges of money laundering (which he survived).

But the king’s first foreign trip, a three-day visit to Germany, with the queen consort, Camilla, won rapturous headlines. Speaking to the Parliament in Berlin, Charles, who has German forebears, switched seamlessly from English to German as he stressed the solidarity between Britain and Germany in defending Ukraine from Russia.

His choice of Europe was no accident. It was clearly intended, in the wake of Britain’s exit from the European Union, to build on a diplomatic rapprochement between Mr. Sunak and President Emmanuel Macron. Visiting Germany in 2020, Charles said, “No country is really an island” — the closest he ever came to publicly criticizing Brexit.

Still, the king’s “soft power” has its limits, even in the Commonwealth. Britain’s former colonies are increasingly chafing at the monarch’s role as their head of state, and with the death of the much-admired queen, Jamaica and others are determined to throw off their links to the royal family.

Nor is Charles ever likely to match the popularity of his mother at home. In a YouGov poll in early 2023, the queen, who died last September, was viewed favorably by 80 percent of respondents. Charles was viewed favorably by 55 percent, putting him behind his sister, Princess Anne; William; and his daughter-in-law, Catherine.

Those numbers are well ahead of most British political leaders, and far better than they were in 1996, after Charles’ failed marriage to Diana, Princess of Wales. At that time, his public reputation crashed so badly that many Britons said they would prefer to see the crown skip a generation to William.

But Charles continues to be saddled with fallout from the palace’s bitter split with Harry and his wife, Meghan, which was stoked by Harry’s memoir and its tell-all accounts of the family’s quarreling.

“The revelations about the king’s relationship with his second son have overshadowed what the king is trying to do in the United Kingdom,” said Ed Owens, a historian who writes about relations between the monarchy and the media. “The tabloid press has been preoccupied by this story of celebrity rather than the more difficult question of how the monarchy is going to evolve.”

Some say the rift with Harry and Meghan deprived the royal family of its last, best chance to modernize its image. While William and Catherine remain popular, Ms. Mayer noted, they are edging into middle age, no longer progressive figures but parents who embody tradition and conservative values.

Charles has yet to make a strategic move to define his reign, royal watchers said. In his public appearances, he remains the same figure he was as Prince of Wales — more down-to-earth than the queen, more apt to dwell on subjects that seize his imagination, like the export of grain out of Ukraine, which dominated a visit he made last year to a charity that helps resettle Ukrainian refugees.

The oath the archbishop announced last weekend, termed the “homage of the people,” grew out of an effort by the palace to make the coronation more relevant and ecumenical — no mean feat for a ceremony whose rituals go back to the crowning of King Edgar in A.D. 973 in the Roman city of Bath.

Leaders of non-Christian faiths like Judaism, Buddhism, and Sikhism will present the king with items of regalia and will greet him before he leaves Westminster Abbey. The archbishop will offer a preamble that nods to other religious traditions.

By encouraging the public to take part in a ritual once reserved for nobility, the palace was clearly hoping to open up the ceremony. It also would showcase the breadth of public support for Charles. But at a time when young people are tuning out the monarchy, expecting them to swear an oath to a king seemed out of touch.

“There’s an appetite for change,” Ms. Mayer said, “and they’re still trying to do business as usual.”

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