Mr O. Panneerselvam, 66, is known as a low-key politician with a penchant for public demonstrations of loyalty to the late Ms J. Jayalalithaa.
In 2014, when Ms Jayalalithaa, then the powerful chief of the All India Anna Dravida Munnetra Kazhagam (AIADMK) party, went to jail for 22 days in a graft case, it was OPS - as Mr Panneerselvam's colleagues call him - who stepped in as acting chief minister.
He wept as he was sworn in back in 2014, kept her photograph in his pocket and refused to sit in her chair to show his loyalty.
Mr Panneerselvam, only two to three years younger than his party boss, has even been photographed prostrating himself before her.
The eldest of eight brothers, Mr Panneerselvam was born into a farming family in Periyakulam in the Indian state of Tamil Nadu. He studied economics at the Haji Karutha Ravuthar Kavuthiya Arts College in Uthamapalayam in Tamil Nadu and later opened a tea shop.
In 1969, at age 18, he joined Dravida Munnetra Kazhagam, a Tamil party, as a party activist. Four years later, he joined AIADMK, started by actor-turned-politician M.G. Ramachandran, and handed the shop to one of his brothers.
In a succession fight following Mr Ramachandran's death in 1987, Mr Panneerselvam first supported Mr Ramachandran's wife Janaki, before switching sides to support Ms Jayalalithaa, the eventual winner.
Like his party rival, Ms V.K. Sasikala, he belongs to the Thevar community and has helped the party woo the community, which forms an important voter base in several constituencies in southern Tamil Nadu.
He rose through the ranks in part because of his devotion to Ms Jayalalithaa and his proximity, ironically, to Ms Sasikala's nephew, Mr T.T.V. Dhinakaran, who is influential in the party.
Mr Panneerselvam, who won his first assembly election in 2001, has held ministerial positions in finance and public works.
After the death of Ms Jayalalithaa at age 68 on Dec 5 last year, Mr Panneerselvam, who wept on Indian Prime Minister Narendra Modi's shoulders at her funeral, once again stood in as acting chief minister.
After resigning from his post to make way for Ms Sasikala on Feb 5, he surprised many when he announced days later that he had wanted to continue as chief minister and had been forced to resign.
Known to be humble and a man of few words, he is now addressing multiple press conferences as he tries to wrest control of the party.
His opponents have accused him of being a traitor while supporters insist he has the requisite experience to lead Tamil Nadu, adding that he was the man the late party chief had turned to when in trouble.
As acting chief minister in the past two months, he has successfully handled relief operations for a cyclone that hit Tamil Nadu.
He was also at the forefront of efforts that helped to overturn a court ban on a "bull-taming" contest - a ban that had sparked protests as young people in Tamil Nadu saw it as an assault on Tamil culture.
He is said to have built up a rapport with Mr Modi and his Bharatiya Janata Party, which is keen to make inroads into Tamil Nadu.
Yet many believe the politician waited too long to stake his claim.
"The lateness of his revolt is surprising. He has made it more difficult by asserting his position (after) having resigned and proposing Sasikala's name," said Jain University pro vice-chancellor and political scientist Sandeep Shastri. "There is a credibility gap."
While Mr Panneerselvam, a father of three, faces an uphill task in becoming Tamil Nadu chief minister again, he is clearly not giving up without a fight.
"The leader of Tamil Nadu should be someone who has been chosen by the people. If the party workers and the people of the state are with me, I will take my resignation back," he said.