Politician Ruangkrai Leekitwattana is known for his skill in scrutiny and earned his "Jack the Giant Slayer" nickname when his petition led to the ouster of the late Samak Sundaravej as prime minister in 2009.
The high-profile petitioner recently sought to reprise that role when, shortly before the May 14 general election, he asked the Election Commission (EC) to investigate Move Forward Party (MFP) leader and list-MP candidate Pita Limjaroenrat for owning shares in a media firm.
His crusade against graft has seen him target key public figures including politicians and state officials, dating back more than two decades, but it was a tax dispute with the Revenue Department in 2006 that put Mr Ruangkrai in the media spotlight.
He was ordered to pay tax on shares received from his father who transferred them outside the stock market. Mr Ruangkrai paid the tax but the department later notified him that the purchase was in fact not liable under the Revenue Code and sent him a tax refund cheque.
At that time then prime minister Thaksin Shinawatra was under fire after members of his family sold Shin Corp shares in a 73-billion-baht deal to Singapore's Temasek Holdings. The Shinawatras did not pay tax on the transaction.
Mr Ruangkrai, who graduated with a bachelor's and a master's in accountancy, used the case to highlight a double standard and his work caught the eye of then auditor-general Jaruvan Maintaka, a graft-buster who invited him to work as her adviser. Following the 2006 coup he was appointed as a senator and was part of the "Group of 40" senators.
His relationship with Khunying Jaruvan took a twist when the latter refused to retire at the age of 65 despite a Council of State ruling calling on her to step down.
Citing an order from the Council for National Security, the 2006 coup-maker, Khunying Jaruvan said she should continue in the job until a new auditor-general was selected as her departure would cause technical and legal hiccups to the office's work.
Mr Ruangkrai asked the Ombudsman to investigate Khunying Jaruvan's status and petitioned the National Anti-Corruption Commission to look into her assets.
But his signature achievement was the downfall of Samak.
He filed a case with the Constitutional Court in 2008, accusing the late prime minister of having a conflict of interest when he hosted a TV cooking show. The court ruled in his favour and removed Samak from office.
In 2010 Mr Ruangkrai was spotted at gatherings of red-shirt members who were supporters of Thaksin. He became a list candidate for the Pheu Thai Party in the February 2014 election which was later nullified by the Constitutional Court.
The 62-year-old political activist kept a low profile after that and returned to Pheu Thai when Khunying Sudarat Keyuraphan served as the party's chief strategist. After a change of guard in the party, he was dropped from consideration to join a committee vetting the 2021 budget bill.
Mr Ruangkrai quit Pheu Thai, and served as a member of the budget scrutiny committee under the quota of the Seri Ruam Thai Party.
To many people's surprise, he defected to the Palang Pracharath Party (PPRP) and worked on the committee vetting the 2022 budget bill.
Prior to his defection, he petitioned the NACC to look into a collection of amulets held by Prime Minister Prayut Chan-o-cha and luxury wristwatches belonging to PPRP leader and Deputy Prime Minister Prawit Wongsuwon.
His current campaign is being viewed by observers as politically motivated to block the MFP leader's bid to become the prime minister and thwart the party's efforts to lead the next coalition government.
Mr Ruangkrai shrugged off the criticism, saying he had obtained information about Mr Pita's shareholding and spent weeks checking the facts before lodging the petition.
"Is anyone else known to own media shares? Mr Pita's name came up, so we must look into it. I'll do the same with politicians from other parties. But to my knowledge there are none," he said.
The MFP leader can sue him for making a false complaint if the allegations prove groundless, he said.
Asked about him switching political camps, the political activist said he puts the law before party affiliation, and he had resolved to maintain and observe righteousness since he first joined the civil service in 1984.
Mr Ruangkrai said scrutiny of political figures including Mr Pita is based on criteria set out in the 2017 charter.
"That's how I work. It's not the right thing to turn a blind eye to those on my side and go after my rivals. My fellow senator complained when I lodged a complaint against him, so I reminded him I had already warned him to behave.
"But I'm not the one who decides their fate. It's the court," said Mr Ruangkrai.