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Malaysia's former authoritarian leader Mahathir Mohamad wasn't invited to the forum that planned to debate whether at well past 90 years he was too old to be prime minister again. He turned up anyway.

"As far as health is concerned, I am not senile yet," he announced to the stunned participants, maybe of whom stood and snapped photos with their smartphones.

For good measure, Mahathir sat in the front row and tweeted: "I'm here guys. Say it to my face."

The recent move was trademark Mahathir. During more than two decades of strong-arm rule, he rarely shied from aggressively confronting opponents, real and perceived. And at 92 his appetite for political brawling remains apparently unsated.

The difference is that Mahathir's targets aren't those of yesteryear such as an imagined Jewish conspiracy against the Muslim world or the domestic opponents he ruthlessly silenced or imprisoned.

In an unlikely comeback, he's switched sides in Malaysian politics, coming out of retirement to unite an opposition that's seeking to end his former party's 60-year hold on power and oust his protege, Prime Minister Najib Razak, in May 9 elections.

Mahathir, a maverick in the early days of his political career who survived expulsion from the dominant United Malays National Organization party, was the first commoner to become prime minister of Malaysia. Though credited with transforming the Southeast Asian backwater into a modern economy, his dominance, like that of his contemporary, Lee Kuan Yew in Singapore, was stifling.

Under his rule, the judiciary was a tool of the government, the media was muzzled and a system of economic privileges for the Malay majority remained entrenched. His retirement in 2003 was welcomed by many Malaysians who wanted the country's progress to be measured more by more than just GDP figures.

Despite that mixed legacy, Mahathir is now welcomed by many "as a messianic figure, someone who has come back from the past" to redeem the country, said Rashaad Ali, a researcher at the S. Rajaratnam School of International Studies in Singapore.

Even after stepping down, he remained influential, smoothing Najib's ascent to the top in 2009 after criticizing his first successor Abdullah Ahamad Badawi for poor economic management and also supported Najib in 2013 elections.

Then he became Najib's most ferocious critic as the prime minister, initially regarded as a cautious reformer, turned to Mahathir's authoritarian playbook to survive a multibillion-dollar corruption scandal at the 1MDB state fund that damaged the country's reputation abroad.

The U.S. and several other countries are investigating allegations of cross-border embezzlement and money laundering at 1MDB, which was set up and previously led by Najib to promote economic development but accumulated billions in debt. The U.S. Justice Department says at least $4.5 billion was stolen from 1MDB by associates of Najib and it is working to seize $1.7 billion allegedly taken from the fund to buy assets in the U.S. Najib has denied any wrongdoing.

Analysts say the unfair electoral system that Mahathir himself helped maintain means the opposition has only a slim chance of gaining a majority in parliament, even if it wins the most votes as it did for the first time in 2013.

But Mahathir could wound Najib, 64, by denying him a decisive victory. An ethnic Malay nationalist, Mahathir by being part of the opposition has lessened the potency of the Malaysian ruling class's perennial race card: playing on fears an opposition win would allow the Chinese minority to dominate Malays.

Despite two coronary bypass surgeries, Mahathir keeps up a hectic campaign schedule. Sometimes he speaks several times a day at rallies that often last till the night and has also updated his tactics for the social media age.

He takes questions on weekly Facebook webcasts to reach younger voters. He has written poignant poems and produced short videos, telling Malaysians he only has a short time left and urging them to vote and save Malaysia from a venal elite.

"I am already old. I am past 90. I don't have much time but within my means, I will try my very best to work together with all my friends to rebuild our nation," Mahathir said in a recent video.

He looks remarkably robust but opponents are quick to seize on any sign of physical weakness and often try to portray him as tired and incapable of leading.

In February when he was hospitalized for a chest infection, he tweeted a photo of himself undergoing treatment.

"Taking a short break," it was captioned. "The fight will go on."