The National Reform Steering Assembly (NRSA) has questioned the merits of new election rules for both MPs and senators in its two-day debate on the draft charter.
Meanwhile, academics warned about empowering the Constitutional Court which may cause further political crises.
Seree Suwanpanont, chairman of the NRSA's committee on politics, cast doubt over the single-ballot system and indirect election of senators proposed by the Constitution Drafting Committee (CDC).
He warned that while it would take some time to see if the new poll regulations work out as designed, the new method had some obvious shortcomings.
The proposed single-ballot system did not reflect voters' intentions, according to Mr Seree. Under the rule, party-list seats would be allocated based on votes cast for constituency candidates, so small- and medium-sized parties that did not field constituency candidates would be at a disadvantage.
He suggested a trial period for MPs elected under the new system, consisting of a term of only two years.
As for the proposal for the indirect election of 200 senators elected from 20 professional groups, 10 from each group, he said the regulation would lead to "organised block votes".
He said the Senate should be wholly appointed but stressed that mechanisms must be in place to ensure the chamber was inclusive and represented diverse groups of people.
A clause requiring senators not to have been members of political parties for 10 years is impractical and would keep numerous "able" candidates out, he said.
Mr Seree expressed concerns about conditions of and requirements for seeking charter amendments, saying that making constitutional amendments too difficult would create a deadlock that would result in the charter being torn up.
NRSA member Kamnoon Sidhisamarn echoed Mr Seree's concerns about the single ballot which he said would deprive small- and medium-sized parties of the chance of getting seats.
He also questioned the rule governing the nomination of a prime minister, saying it failed to say when the House should elect the premier and he wanted to know the reason why.
Witthaya Kaewparadai, also an NRSA member, said the party-list system should be scrapped because it has so far failed to bring in newcomers without political backing to the arena. The party list was instead exploited by party financiers to gain entry into politics.
The NRSA was scheduled to deliberate the draft charter Monday and Tuesday, and then present its "suggestions" to the CDC by Monday.
The CDC is required to complete the final draft by March 29, in line with a 180-day time frame stipulated by the interim charter.
Meanwhile, academics have decried the draft charter, arguing that it will shift the balance of power in favour of the state apparatus and limit political representation.
They also criticised the extended powers given to the Constitutional Court. Chulalongkorn University lecturer Porson Liengboonlertchai warned that the powers given to the court to impeach holders of political positions are detrimental to judicial institutions and may cause further political crises.
"In the past, the Constitutional Court did not even have the direct means to impeach a political representative, yet we have seen what trouble it brought," he said.
The drafters' decision will transform the Constitutional Court into a political actor, Mr Pornson argued. This may affect all other courts as well, which might see their rulings contested in the future. Countries in which the authority of the court is questioned are countries at risk of political collapse, he added.
In an effort to curb corruption, the charter drafters deliberately weakened political parties and the role of parliament, said Siripan Nogsuan Sawasdee, a political science lecturer at Chulalongkorn University.
"The result is a draft which limits participation, limits the government's authority but gives the state unlimited powers," Ms Siripan said. Political institutions are indispensable in a democracy, she added, as they enable public participation.
If the CDC wishes to impose more checks on political parties, it had better strengthen them, she argued, in order to allow citizens to truly take part in the direction and day-to-day management of those parties.
Ms Siripan also criticised the single-ballot election for both constituency and party systems, arguing that it could pave the way for more vote-buying.
CDC chairman Meechai Ruchupan Monday pledged to consider all the suggestions and make changes where possible. However, he said it was too early for the CDC to say what changes would be made to the draft.
Mr Meechai also pointed out that there were misunderstandings about the draft and threatened action against people, especially politicians, who deliberately distorted the charter's content.
In a related development, the CDC and key government figures on Monday downplayed ousted prime minister Thaksin Shinawatra's criticism of the charter draft, saying it was his personal opinion. Thaksin described the draft as "lousy" and said it would be a step backward for the country.