Pheu Thai heavyweight Chaturon Chaisang has lashed out on Twitter at Prime Minister Prayut Chan-o-cha over his remarks about the possibility of an unelected premier taking the reins after this year's planned election.
Key Democrat Party figures also took aim at the prime minister.
Mr Chaturon said he was convinced that the "outsider" mentioned by the PM in his comments could only be Gen Prayut himself.
On Thursday, Gen Prayut was non-committal over whether he will return as a non-elected outsider prime minister after the general election expected later this year.
Gen Prayut said he has had to avoid speaking clearly on the issue as this would only "impose constraints" on him.
The prime minister said that being an outsider premier is now permitted by the constitution and he predicted that there will be no more coups in the future.
But Mr Chaturon said he regarded this as a veiled threat from Gen Prayut.
"The problem for Gen Prayut is how the people will think of him as an outsider prime minister and how he will gather the support of 250 MPs to back him," Mr Chaturon said.
"If Gen Prayut is to become an unelected premier, I am sure Pheu Thai will become the opposition," according to Mr Chaturon's tweet.
If MPs fail to select a new premier from candidates nominated by party lists, a provisional clause under Section 272 of the charter would trigger the alternative route.
In such a situation, senators can join MPs in proposing a motion to suspend the rule requiring prime ministerial candidates to come from political party lists, paving the way for an "outsider" prime minister to be selected.
Such a motion requires the support of at least two-thirds of both MPs and senators, or 500 of them, to suspend the rule.
However, Col Winthai Suvaree, spokesman for the National Council for Peace and Order (NCPO), yesterday said that Gen Prayut's remarks about an outsider prime minister and the possible end to future coups were misinterpreted by some media outlets.
Col Winthai explained that the constitution is designed to ensure flexibility in the event that a prime minister cannot be chosen from among parties' candidates through a normal parliamentary process.
The constitution drafters only sought to ensure flexibility to help prevent a repeat of the political conflict that culminated in the 2014 coup, Col Winthai said.
Former Pheu Thai MP for Chiang Rai Samart Kaewmeechai yesterday said that it would be very difficult to gather the support of 500 members of both Houses to suspend the rule.
Moreover, the constitution is designed in a way that makes it hard to amend.
"If things eventually reach an impasse, this will only trigger a fresh coup and the constitution will be torn up again," Mr Samart said.
If Gen Prayut wants to be a politician in a democratic system, he should run in the poll rather than come to power via a special method, Mr Samart said.
Worachai Hema, a former Pheu Thai Party MP, said that even if Gen Prayut returns as a non-elected prime minister after the election, it will not be easy for him to run the country as he does now.
He would face resistance from MPs who disagree with him in parliament and the passage of legislation will not proceed as smoothly, Mr Worachai said.
The Democrat Party's deputy leader Nipit Intarasombat said that it is obvious that Gen Prayut will likely return as an outsider prime minister, and he must be ready for scrutiny like other politicians, Mr Nipit said.
Supachai Jaisamut, deputy secretary-general of the Bhumjaithai Party, said his party had no objections if Gen Prayut became a non-elected MP as allowed by the constitution.
Nikorn Chamnong, director of the Chartthaipattana Party, yesterday said that the party was not worried as to whether Gen Prayut would emerge as an outsider prime minister.
It remains to be seen if Gen Prayut will allow himself to be nominated as a non-MP premier, Mr Nikorn said.
He refused to comment when asked if Chartthaipattana would join a government if a military-backed political party won a majority of House seats to form a government.
The Democrats and Pheu Thai have ruled out the possibility of joining hands to prevent the military's return to power after the poll.
The idea of the country's two major parties working together to "reclaim" democracy was floated on Tuesday by former Democrat leader Bhichai Rattakul.
Noppadon Pattama, former foreign minister and a key figure in Pheu Thai, said Pheu Thai is not ready to join hands with any other party, while Ong-art Klampaibul, deputy Democrat Party leader, said that an alliance would never happen as they have opposing political ideologies.
