The Pheu Thai and Democrat parties have joined growing criticism of the so-called "crisis panel", fuelled by fears the panel proposed under the new charter will persuade the public to reject it at a referendum.
The Constitution Drafting Committee (CDC) proposed a clause in the draft charter to launch a national committee on reform and reconciliation - comprising the army, navy, air force and police chiefs - with special powers to intervene in a political crisis by seizing both executive and legislative powers from the government and parliament.
Under the provisional clause, members of the crisis committee, as it is also known, can vote to intervene in a crisis if they believe the government cannot function and national administration is seriously compromised. A two-thirds vote in favour is needed for intervention to take place.
Democrat leader Abhisit Vejjajiva said he did not think the crisis panel would be capable of handling crises. Soldiers and police have in the past found it difficult to control violence during political unrest, he said.
In light of this, the proposed crisis panel is not likely to measure up if a political crisis similar to the one leading up to the May 22, 2014 coup occurs again, he said, adding he did not see the point of setting up the panel because most of its members are already in positions of power.
Instead, certain charter provisions should be allowed to be suspended in a given situation, so if the cabinet fails to handle a crisis, it is forced to quit to clear the way for elections.
Former Pheu Thai MPs Samart Kaewmeechai and Chavalit Witchayasut on Monday handed a letter to the CDC to oppose several proposals in the draft charter, including the one to establish a crisis panel.
Mr Samart criticised the proposed crisis panel as "a state within a state", since it can take the executive and legislative reins at any time it deems necessary.
The new charter should not be designed to promote power that does not come from the people, he said.
"Before, we might have called this an extra-constitutional power. But now it is written in the draft charter.
"If the charter is written this way, it will not pass the referendum. The people will not accept it if their power is taken away from them and all the effort and time spent during the charter-drafting process will go to waste," he said.
He urged the CDC to reconsider the idea before submitting the draft charter to the National Reform Council (NRC) for consideration. The NRC will vote on the draft charter on Sept 7. If the NRC approves the draft, a referendum is expected to be held in January next year where the public will decide whether to accept it.
Sombat Thamrongthanyawong, who chairs the NRC's political reform panel, said the draft charter is fraught with flaws, which put off even those who are demanding an early poll.
The crisis panel is a case in point, he said. If the Pheu Thai and Democrat parties disapprove of the panel, this will influence voters, who make up their support base.
The voters are waiting for "signals" from the politicians they support, he said.
According to Mr Sombat, the scope of power of the crisis panel is similar to that of Section 44 in the interim charter, which goes against the principles of democracy.
Defending the draft charter, CDC spokesman Lertrat Rattanawanich said some provisional clauses under the charter are needed to ensure peace and stability while the country is in a transition towards democracy. The provisional clauses will last between three to five years, he added.
CDC member Paiboon Nititawan shrugged off former prime minister Thaksin Shinawatra's call for his supporters to reject the draft charter.
"It's not surprising that Thaksin views the draft charter as the worst scenario. He doesn't want the constitution to come out this way. What he wants is the abolished 1997 constitution as he would benefit from it," Mr Paiboon said.
He was responding to a speech by Thaksin on Friday, posted on YouTube, urging his supporters to turn down the draft. In the clip, Thaksin is seen telling a gathering of red shirts in Finland that the draft constitution goes against the will of the majority of people.
Mr Paiboon defended the draft charter, saying it reflected the desires of people across the country as reflected in surveys and public hearings.