The Democrats have been thrown into disarray as party heavyweight Thaworn Senneam faces a probe for being "disloyal" by allegedly acting for the benefit of a pro-regime party.
The probe comes as speculation swirls about possible divisions in the country's oldest party following the recent party leadership contest.
The case centres on why Mr Thaworn, a former deputy leader of the Democrats, allowed Suthep Thaugsuban, a co-founder of the Action Coalition for Thailand Party (ACT), to use his house in Songkhla as "an office" for the ACT to accept applications to join the party on Wednesday.
At the time, Mr Suthep was engaged in a campaign walk to recruit members in several provinces.
While in Mr Thaworn's house, Mr Suthep took the opportunity to take aim at the Democrat Party's recent leadership contest which saw Abhisit Vejjajiva re-elected. Mr Thaworn was a key supporter of another contender, Warong Dechgitvigrom.
Mr Suthep said: "Were it not for all the cheating, Mr Thaworn would have become the party's secretary-general."
Mr Suthep also predicted the Democrat Party would lose Constituency 7 in Songkhla and in the three southernmost provinces.
Mr Suthep said that Sirichoke Sopha, a former Democrat MP for Songkhla who is tipped to run in the province's Constituency 7 under the Democrat Party banner, still had low popularity ratings in the constituency and may not win a seat there.
Mr Thaworn agreed with Mr Suthep, saying Mr Sirchoke should heed Mr Suthep's warning so he can make efforts to improve his standing in the constituency ahead of the poll.
Responding to Mr Thaworn's remarks, Mr Sirichoke said: "Mr Thaworn probably said that because he might still have been disappointed after the loss [by Dr Warong] in the party leadership race. But our party has regulations. It remains to be seen if a veteran politician like Mr Thaworn will escape punishment," Mr Sirichoke said.
Democrat deputy leader Nipit Intarasombat said several party members were unhappy and had lodged complaints against Mr Thaworn.
Mr Nipit said that Mr Thaworn will face a disciplinary probe.
A fact-finding panel will be set up to look into the matter and Mr Thaworn will be asked to explain himself.
Mr Thaworn's action may set a precedent to deter party members from allowing other parties to use the Democrat Party's branches for their own political purposes, Mr Nipit said.
He said Mr Suthep's remarks against the Democrat Party were harsh given that Mr Suthep was formerly its secretary-general.
Mr Nipit added that Mr Suthep had made it clear during his campaign in the three southern border provinces that the Democrat and ACT parties were rivals in the upcoming general election.
"In light of this, was it right to let a political rival use our house to recruit members?" Mr Nipit said.
A party source said Mr Thaworn's actions were inappropriate because he has expressed his wish to run as an MP candidate in Songkhla under the Democrat Party banner.
Helping a rival party would be a breach of Democrat Party regulations, the source said.
Many Democrat members are now confused as to where Mr Thaworn's loyalties lie, the source added.
Mr Abhisit said he has asked Mr Nipit to look into the matter. If Mr Thaworn violated party regulations, the party will consider taking action against him, he added.
Mr Abhisit insisted the probe has nothing to do with Mr Thaworn supporting his challenger, Dr Warong, in the party leadership race.
Mr Thaworn said he was ready to face the probe and accept punishment if the party decides he was in the wrong.
However, Election Commission secretary-general Jarungvith Phumma said yesterday that Mr Thaworn's action could not be seen as one that allowed other parties to control its affairs -- an offence that could lead to the party being dissolved under the law on political parties.
Pol Col Jarungvith described Mr Thaworn's behaviour as "an act of personal generosity toward others".
Mr Nipit brushed aside talk that the probe against Mr Thaworn would drive a wedge deeper into a perceived rift in the Democrat Party.
Mr Thaworn, a six-time MP for Songkhla, threw his support behind Warong Dechgitvigrom, a former Democrat MP for Phitsanulok, in the party leadership race.
A party source said Mr Thaworn expected to be named secretary-general if Dr Warong was elected party leader.
Mr Thaworn still has considerable political clout, making him an important part of the Democrat base.
He also enjoys close ties with Mr Suthep.
Mr Thaworn was one of a number of former Democrat MPs who left the party to join the now-defunct People's Democrat Reform Committee led by Mr Suthep, who also quit the party to lead the street protests against the Pheu Thai administration in late 2013, which culminated in the 2014 coup.