Mahathir, 90, is Malaysia’s longest-serving prime minister who stepped down in 2003 after ruling for 22 years.

Since last year, Mahathir has been calling for Najib to step down over the scandal-hit state-owned investment firm 1 Malaysia Berhad (1MDB) which racked up 50 billion ringgit of debt over six years. Najib chairs 1MDB advisory board.

Mukhriz, 51, is a politician with the ruling Umno party. But he told the South China Morning Post he

has no ambitions to become Malaysia’s prime minister and emphatically denied the government’s accusations.

Mukhriz, the fourth of Mahathir’s seven children, has an older brother, Mokhzani. Mahathir had publicly forbidden any member of his family to be involved in politics while he was in office.

“It was public knowledge when he [Mahathir] was president of the party and prime minister that he discouraged family members, particularly Mokhzani and myself, from playing an active role in politics and government to avoid accusations of nepotism,” Mukhriz said.

“You would think that if he [Mahathir] had such ambitions for me, he could have easily done it while he was in power. Why wait 13 years after his retirement?

“It’s totally absurd and they [Najib and his supporters] should know him better than that as he [Mahathir] was their boss for 22 years.”

Mukhriz first stood for elections in 2004 for the post of Umno Youth chief in Kubang Pasu in the state of Kedah, which was previously held by his father, and he lost.

“My father told me I would lose and I did. It was a real wake-up call for me, that nothing can be taken for granted. It was a baptism in fire,” Mukhriz said.

Later that same year, Mukhriz won a seat in Umno Youth’s executive council by securing the most votes.

His political star started to climb in 2008 when he won a parliamentary seat in Jerlun, Kedah and he was made deputy minister for international trade and industry in 2009 by Najib.

Softly spoken and unassuming, Mukhriz is described as being closer in character to his gentle mother, Siti Hasmah, a doctor, than his tough, unyielding father.

“I actually don’t have ambition to become Prime Minister because I think it’s a tough job, especially now,” Mukhriz said. “If there is a change in leadership, the new person will be inheriting a mess. I don’t envy the next guy.

“I know what it’s like. I know how tough it is. My father was [prime minister] for 22 years. It’s the kind of job you have to throw in your all and everything into it.”

On Tuesday, The Wall Street Journal (WSJ) reported that US$681 million of funds from 1MDB were remitted to Najib’s personal accounts shortly before the country’s 2013 general elections. Najib’s aide Sarifuddin denied the claims.

“The allegation that the funds were from 1MDB is completely false,” said Sarifuddin, reiterating that the money was a “donation” from the royal family of Saudi Arabia.

“This is a lie that … Mahathir Mohamad has unsuccessfully used to try and unseat a democratically elected government,” Sarifuddin said.

Najib has denied any wrongdoing and the country’s attorney general cleared him of criminal misconduct.

According to Mukhriz, 1MDB is in dire condition.

“If 1MDB does not get resolved quickly and satisfactorily, the nation is going to suffer … we don’t know how long it is going to take to pay back those debts,” he said.

He criticised the sale of 1MDB core assets to “foreign companies” to raise cash, saying there were enough local companies with the financial resources and experience to take over the assets to raise funds.

“For us to lose such premium assets to foreign countries I think it’s a major loss,” he said.

“Monies were borrowed in the billions of ringgit that was meant for development. As we all now know, tens of billions went missing. Now we have to sell those assets to pay back debts ... It’s a double whammy. We are paying off debts and at the same time we have lost those assets.”

The assets were bought by Chinese state-owned enterprises at the end of last year.

Mukhriz is also the former chief minister of the northern Kedah state where he served from 2013-16. He stepped down in February after announcing he had lost the state legislative assembly’s majority support, due to a concerted movement by Najib’s camp to oust him while trying to muzzle him from speaking out on various national issues.

Some observers concluded Mukhriz was merely a casualty of the row between Najib and Mahathir.

According to Mukhriz, after his removal, Najib offered him a position as a Deputy Minister where he would be promoted within a few months to full ministership. No portfolio was mentioned but Mukhriz declined the offer.

“I felt it would be hypocritical of me to criticise [Najib] on one hand and to take up his offer on the other,” Mukhriz said.

While Najib is too entrenched in the system to be removed – general elections are not due until 2018 and he retains power due to the support of Umno’s 191 division chiefs – he is believed to have lost significant grass-roots support.

“I think only 20 per cent of Umno’s 3.4 million members support him,” Mukhriz said.

As criticism mounts against Najib, he has cracked down on dissidents and blocked at least two critical news portals.

Mahathir himself is subject to four police investigations. But the former leader is undaunted, giving a series of speeches in public forums.

He recently launched a Save Malaysia campaign to collect 1 million signatures to pressure the country’s sultans, the traditional rulers, to force Najib to step down.

Asked about the state of Mahathir’s health, Mukhriz said he is “fine”.

“He is almost 91, but his health is fine,” he said. “He is driven, he has a mission so this is not the time to give up halfway. He inspires much younger people to do something about our lot and not just leave it to people whom we longer trust.”