“Lots of people feel that Najib has destroyed much that has been built for this country,” he said.

“People are calling our leader a crook. That is not something I would like to see perpetuated. It must change back to the days when we were doing well.”

Najib has sacked critics in his own government including an attorney general and deputy prime minister and muzzled the media since the corruption scandal erupted two years ago.

The US and several other countries are investigating allegations of cross-border embezzlement and money laundering at 1MDB, a state investment fund set up and previously led by Najib to promote economic development but which accumulated billions in debt.

Najib has denied any wrongdoing.

Mahathir said he doesn’t want to be prime minister again and that his aim is simply to topple Najib and restore Malaysia’s reputation.

He keeps a hectic schedule, criss-crossing the country to speak at political rallies. He has undergone two heart bypass surgeries but shows no sign of slowing down.

Despite speculation that Najib may call elections this year, Mahathir said he believed polls would be held in 2018 because Najib wanted time to strengthen his support in eastern Sabah and Sarawak states on the island of Borneo.

The two states, both strongholds of rural Malay support for the governing National Front coalition, jointly contribute about a quarter of parliamentary seats and helped Najib win the 2013 polls despite losing the national popular vote to the opposition for the first time.

“We will win support on the ground,” Mahathir said.

“We hope to win with a simple majority.”

Yet gerrymandering will make it tough to win more seats than the ruling coalition. The Election Commission last year redrew electoral boundaries that critics said created more Malay-majority seats to ensure victory for Najib though opposition parties are contesting it in court.

“Whatever may be the differences in the past, the problem that we face with Najib is far greater. If we don’t get rid of him, we will achieve nothing,” he said.

Mahathir said he once thought Najib would be admired like his father – Malaysia’s second prime minister Abdul Razak Hussein – but Najib went “from bad to worse” with the 1MDB scandal.

It turned the former mentor into Najib’s most strident critic.

Mahathir often voiced his criticism on his blog, Chedet.cc, taken from a childhood nickname given by his sisters, and attended a massive street rally in Kuala Lumpur last year that called for Najib’s resignation.

Mahathir then formed the Bersatu party together with Najib’s former deputy Muhyiddin Yassin, who was sacked last year for questioning the premier about the scandal.

Even more dramatically, Mahathir this year persuaded his former political nemesis, including opposition icon Anwar Ibrahim who was imprisoned on Mahathir’s watch, to accept Bersatu into Anwar’s opposition coalition.

Anwar was Mahathir’s deputy until he was sacked in a power struggle in 1998 and later imprisoned on charges of corruption and sodomy that Anwar said were trumped up. Anwar was freed in 2004 but in 2015 he returned to prison following a second sodomy conviction that critics said was a political conspiracy to break up the opposition.

Mahathir’s comeback is a boost to the opposition that has been plagued by infighting. His popularity with ethnic Malays may also make the opposition more palatable to those voters, who are the bedrock of Najib’s ruling coalition.

Recently, the alliance announced they would contest elections under a single logo and have a common constitution.

Mahathir said there was a deep reservoir of anger among urban voters, and while the rural poor might not feel affected by the graft scandal, they were upset by a higher cost of living that partly stems from an unpopular goods and services tax.

“This country has the capacity to become a fully developed country if the right policy and plans are carried out,” he said.

But under Najib, we “run the risk of becoming a bankrupt country, a failed country.”