Key Democrats expressed shock over the defection of Thawil Praison to the Move Forward Party (MFP), acknowledging the country's oldest party has lost one of its foremost legal experts.
Mr Thawil, former chief of the Democrats' legal team and ex-deputy House speaker, declared on Tuesday he had quit the party over its procrastination in decentralising political power.
It was a decision he'd thought over for some time, he said.
Democrat spokesman Ramet Rattanachaweng on Wednesday expressed dismay over Mr Thawil's departure and said he was one of the best legal experts he'd worked with.
"I am shocked and saddened," the spokesman said in a Facebook post. "Almost all draft laws had to go through Mr Thawil because of his expertise."
Mr Thawil said that before the 2019 general elections, the party assigned him to look into decentralisation policies but afterwards appeared to lose interest in the matter. Most MPs lived in the provinces and he rarely had the opportunity to discuss the distribution of power with them, he said.
When he met Nattacha Boonchaiinsawat -- the Move Forward MP for Mr Thawil's old Bangkok district, Bang Khunthian -- they found their views on decentralisation policy struck a chord, Mr Thawil said. He then decided to join the MFP, although he will neither assume a party position nor contest elections.
"I have no conflict with the Democrat Party," Mr Thawil said, adding he remained on amicable terms with his old party and its members.
Mr Thawil said power decentralisation is the only issue he has in common with the MFP, and said he had no concerns he might be viewed as supporting a movement aligned with certain political groups, a reference to monarchy reform advocates.
The veteran politician said he also talked with Piyabutr Saengkanokkul, the Progressive Movement secretary-general, about the party's plans to achieve decentralisation by amending the constitution.
Mr Piyabutr on Tuesday tweeted that Mr Thawil had agreed to join the nationwide campaign and help set up an academy to promote the decentralisation issue.
"I warmly congratulated Mr Thawil on his decision to take part in the Progressive Movement campaign," Mr Piyabutr wrote. "He has also applied for lifetime membership of the Move Forward Party without demanding he be able to run for election as an MP." .
Det-it Khaothong, a Democrat MP for Songkhla and deputy party leader, said on Wednesday he was puzzled by Mr Thawil quitting the party and moving from one end of the ideological spectrum to the other. He also downplayed the significance of the defection, saying more people were joining the party than leaving it.