The coalition government, whose version of the charter amendment draft is expected to receive support from parliament today, has called for senators to back it in order to defuse potential political conflict.
In presenting the government's bill on Wednesday, chief whip Virat Rattanaset and Bhumjaithai Party MP Supachai Chaisamut emphasised the need to amend the constitution by setting up a new team of charter writers, as a way out of the political conflict dividing the country.
Mr Virat, a Palang Pracharath MP, said backing from the Senate was needed to pass the government's bill in the first reading.
The two key government MPs made it clear the coalition wanted the first two chapters of the constitution left untouched.
"The charter amendment must not touch Chapter 1, on general provisions, and Chapter 2, on the monarchy," Mr Supachai said during a joint session of parliament.
Part of the first chapter and the entire second chapter of the constitution lay out the power and role of the monarch.
Pheu Thai Party and opposition leader Sompong Amornvivat said the opposition had submitted amendment bills which were intended to steer the country towards democracy and revamp an election system they deemed unfair to parties and which caused confusion.
Chief opposition whip Sutin Klungsang told the Bangkok Post that Pheu Thai, which has proposed five drafts, may lose out to the government's draft due to inadequate support.
The government has enough senators on its side to support its version.
For the motion to pass, a majority of the 750 members of both houses is needed and this must include one-third of the 250 senators (84). After the vote, a 45-member committee will be set up to scrutinise the bills.
The vote, which is scheduled to begin at 6pm, is expected to take up to four hours to complete. Both chambers will vote, with each member being called by name to say whether they supported or rejected each version of the draft amendments.
According to Mr Sutin, the Constitution Drafting Assembly of Thailand (CDA) would spend at least 24 months considering the government's version, while the opposition's drafts could mean a new constitution being rewritten inside 15 months.
"I believe it [the process] will take as long as possible," he said.
A member of the Free People Movement performs a mock ritual in which senators appointed by the coup-makers are subdued and confined in a jar during a small protest outside the army's headquarters on Ratchadamnoen Avenue in Bangkok. (Photo by Somchai Poomlard)
Parliament is likely today to choose a draft as the basis for the charter rewrite when it votes on the six amendment drafts likely to come before it, according to a government source.
While Prime Minister Prayut Chan-o-cha has yet to send a signal on how the 250-military appointed Senate should vote, there is a solid chance that government MPs and senators will support the amendment bill put forward by coalition parties.
The source said the passage of the charter amendment bill in the joint sitting would serve as a gesture to the student-led movement that both the government and parliament were paying heed to their call for charter changes, said the government source.
Student activists plan to gather again today outside parliament to put pressure on lawmakers to support charter amendments.
Members of the Free People Movement on Wednesday turned up outside the army's headquarters and urged military personnel to give up their roles as senators.
The government version of the bill targets Section 256 of the 2017 constitution to make way for the setting up of a 200-member CDA with the first two chapters of the constitution left untouched.
Six charter amendment bills will be examined by the joint sitting of MPs and senators.
Two vouch for the setting-up of the CDA; the other four deal with the role of the Senate and the orders of the now-defunct National Council for Peace and Order.
Meanwhile, the "Thai Pakdee" group, an ultra-wing group led by former Democrat MP Warong Dejkitvigrom, on Wednesday submitted a petition to the House Speaker and the Senate Speaker containing the names of 130,000 people opposed to the charter amendment bid.
Mr Warong threatened to seek the intervention of the Constitutional Court if parliament had a new charter drafted.
He said the 2017 charter passed a national referendum and parliament could only make changes to it, not replace it.