Mahathir was named the opposition's prime ministerial candidate yesterday after weeks of infighting, in an extraordinary turnaround as his heirs in government face a massive financial scandal.
He will be the world's oldest leader if the coalition backing him wins a general election due by August, although analysts believe this is unlikely.
The authoritarian leader known for his acid tongue ruled the country for 22 years until 2003, making him Malaysia's longest-serving premier. He jailed opposition members without trial on security grounds in 1987 and was seen as an authoritarian figure who trampled over human rights.
The US Justice Department alleges USD 4.5 billion was stolen from the investment vehicle, 1MDB, in a campaign of fraud and money-laundering. Najib denies any wrongdoing and has cracked down hard, purging critics from his government and curbing domestic investigations.
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The most remarkable aspect of Mahathir's return to frontline politics has been a rapprochement with his former nemesis and jailed opposition leader Anwar Ibrahim, a dramatic shift after the pair's falling out dominated the political landscape for two decades.
After being released Anwar led the opposition to its best-ever showing in 2013 elections, but was imprisoned again in 2015 under Najib's government. Anwar has condemned his convictions as politically motivated.
After Mahathir cut ties with ruling party the United Malays National Organisation (UMNO) he sought to mend his broken relationship with Anwar, with the pair meeting in 2016 for the first time in 18 years.
His party, Bersatu, joined the main opposition coalition Pact of Hope, which includes Anwar's party and many other former sworn enemies, and yesterday he was endorsed as the grouping's candidate at their convention.
The opposition hopes that Mahathir will boost its chances with Muslim Malays, who make up over 60 percent of the population, with the rest comprising mainly ethnic Chinese and Indians.
Tian Chua, a lawmaker from Anwar's People's Justice Party, said Mahathir could deliver the knock-out blow to a government already reeling from the 1MDB scandal.
But ruling party MPs quickly dismissed his candidacy and social media was flooded with angry comments criticising the decision to put forward a man accused of ruling with an iron fist.
"It is ridiculous. He is over 90 and he started all the problems we are facing," said Facebook user Radha Dulip Singh.
Analysts said a victory for Pact of Hope looked unlikely. A survey by pollster the Merdeka Centre last month showed the ruling Barisan Nasional (BN) coalition potentially regaining its crucial two-thirds parliamentary majority, needed to amend the constitution.
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