The fierce battle for power between Malaysia’s former prime minister Mahathir Mohamad and his one-time protege Najib Razak, the current leader, will be the undoubted headline act this election year.

But in the background, the bitter feud between 92-year-old Mahathir and the country’s hereditary Malay monarchs – stretching back to his strongman rule from 1981 to 2003 and reignited last year – is likely to be cause for some political fireworks too, observers say.

In December, Malaysians were given yet another a glimpse of the bad blood between the nine provincial sultans – who take turns being “king of kings” – and Mahathir, after one of them slammed him for his comments disparaging the Bugis community who make up part of the country’s majority Malays.

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The rebuke from Sultan Sharafuddin Idris Shah of the wealthy Selangor state was not the first royal tongue lashing Mahathir came under in 2017.

Last January, the influential Sultan Ibrahim Ismail, ruler of the Johor state, criticised Mahathir for “playing the politics of fear and race” after the former premier raised questions about mainland Chinese investments in the province.

Sharafuddin, who delivered his missive through a rare media interview with The Star newspaper, said Mahathir’s wry comments about the Bugis community – which Najib belongs to – were divisive.

“He is an angry man and will burn the whole country with his anger,” the sultan said. The monarch said his sentiment was shared by all nine hereditary sultans.

Mahathir did not apologise on both occasions, and later quipped when asked by reporters about the monarch’s comments: “Yes, I am a very angry man, you can see how angry I am. I will burn you, I am always burning things.” Following the latest stand-off, he and his wife Siti Hasmah Mohamad Ali returned decades-old titles they had received from the Selangor palace.

The pattern of increasing hostility between both sides is likely to continue in 2018, observers say.

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“There will be a continuation of such on-and-off skirmishes [between Mahathir and the sultans] all the way to the next general election and beyond,” Oh Ei Sun said, a Malaysian political analyst.

Ahmad Marthada Mohamed, a politics professor at the University of Northern Malaysia, said one factor contributing to the sultans’ becoming “very vocal” was that ordinary citizens have increasingly taken to petitioning them to intervene in politics, even if that is not allowed in the constitution.

“These rulers can speak their mind without any fear of retribution compared to regular citizens who might be charged in the court of law if what they say touches on sensitive issues such as race, religion or politics,” Ahmad Marthada said.

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Mahathir’s anti-Najib campaign, which he claims is necessary to rid the country of a “kleptocracy”, or thieving government, is seen by the sultans as a threat to their own special status, according to political insiders. “They [the rulers] would obviously rather the status quo remain. Dr Mahathir’s defection [to the opposition] has injected serious uncertainty to the political landscape, and the sultans’ comments show they are not pleased with that,” said an opposition politician who declined to be named, citing the sensitivity of commenting about the monarchy.

Unlike its northern neighbour Thailand, Malaysia does not have lese majeste laws that punish those who offend leaders, but the country’s colonial-era sedition laws have been used in the past against royal critics.

While constitutionally required to remain above the political fray, the sultans have traditionally been close to the United Malay National Organisation (Umno), which is helmed by Najib.

The ruling party has governed the multiracial country for six decades under a social contract in which Malays wield the most political power. Mahathir quit Umno in 2016 and joined arch rivals in the opposition in a bid to oust Najib in the upcoming polls.

Current politics aside, some observers think the sultans’ antipathy towards the country’s longest-serving prime minister date back to his efforts to clip their wings during his political heyday.

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“Mahathir’s palace problems are a hangover from his ‘tough love’ approach towards the sultans during his days at the top,” Joceline Tan, a prominent political columnist, wrote last month.

The former premier’s first major run-in with the sultans was in 1983, when he forced them to give up the right to veto new laws by withholding assent.

A decade later, in 1993, he further curtailed their powers by ending their immunity from prosecution following complaints of errant behaviour.

In his interview with The Star, Sharafuddin dismissed the notion that the monarchs were exacting revenge. “No, we do not believe in revenge. God is great. He will show to the people. Back then my father told me, today is his [Mahathir’s] day, tomorrow will be our day,” Sharafuddin said.

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Mahathir, who served seven Yang di-Pertuan Agongs – the paramount king position rotated every five years among the nine sultans – has previously insisted that, despite his royal run-ins, he is not anti-royalist. “If the sultan doesn’t like me, it’s alright. I am not asking people to like me, I am just going to stand up for what is right,” he told This Week in Asia in an interview last March, when asked about the harsh words Sultan Ibrahim Ismail had aimed at him two months earlier.

Oh, the political watcher, said the slow-simmering feud was unlikely to be a major factor in the impending polls, in which Umno and its allied parties are seen as favourites despite the challenge mounted by Mahathir.

And in the court of public opinion, the revered monarchy is also unlikely to come up second best against Mahathir, other observers say. “The Malay ruler continues to be seen as a guardian of the people’s interest ... and a symbol of national unity that is above politics,” said Awang Azman Awang Pawi, a professor of Malay Studies at the University of Malaya.