Democrat party leader Abhisit Vejjajiva has urged draft constitution writers to rethink senator origins, accommodation of non-MP prime ministers, the single ballot system and the retention of special powers by the junta's leader after the new constitution takes effect.
The former prime minister wrote on Facebook on Friday he welcomes more stringent anti-corruption measures but checks and balances among all bodies must be in place.
"For instance, the House speaker, who serves as parliament speaker, should be allowed to apply his discretion in taking action when the National Anti-Corruption Commission abuses its power. This is an example of the loopholes in the draft," he wrote.
On curtailed rights and freedom, Mr Abhisit does not agree with the writers' decision to leave out the provisions.
"Importantly, the draft does nor prescribe how communities can concretely exercise their rights like the 2007 charter did. It also doesn't name independent bodies to assess impacts on the environment and health or one to look after consumers' rights. The right of voters to propose laws is also left out.
"I insist these provisions in previous charters are not obstacles that led to political crises so they should be included in the draft."
Mr Abhisit also urged the writers to reconsider restricted direct political participation by people, which makes the political system less democratic.
"In particular, senators should be directly elected, not handpicked from applicants in each professional group, a method prone to lobbying.
"Although senators will have less power under this draft, they approve nominations of the members of independent bodies, which are instrumental in tackling corruption," he wrote.
The ex-PM also does not agree with a non-MP prime minister.
"Should the need arise to give up the democratic principle that a PM should be MP, clear conditions must be set, such as this can happen only with a special resolution of parliament and only at the time of crises. The door shouldn't be open permanently for persons who do not represent Thai people to head the executive branch.
"Besides, an outsider premier is not neutral since he needs to be nominated by a political party."
While Mr Abhisit has no problem with the mixed-member proportional representation introduced in the draft, he pointed out the will of the people cannot be accurately reflected in the single-ballot election system the writers want to use.
"Besides, competition will be more intense and vote-buying more rampant in the single-ballot system. It will be more complicating as the proportion of political parties change constantly."
Mr Abhisit added there are more provisions that should be reconsidered such as the one that gives the junta's chief power even when the new charter is in effect.
"This could lead to conflicts of laws and challenges of legitimacy. Besides, the reform process outlined in the draft is neither concrete nor clear."
Mr Abhisit wrote his party would submit a letter to the CDC about the proposals.
"The best solution for Thailand right now is to have a constitution that is acceptable to people, which will give it legitimacy and make it sustainable."
He also urged the junta to amend the interim charter so people know what they will get if they vote "no".
The views of Mr Abhisit and his party on the draft are widely watched after several key members of Pheu Thai, the other big party, made clear they would not endorse it.
Mr Abhisit's post on Friday showed the two parties share several common views on the draft.
Since the two parties together made up 83% of votes in the latest successful general election in 2011, the opinions of their key members on the draft are taken as gauges for the success of the draft at the referendum, slated in the middle of this year.
Also on Friday, Pornpetch Wichitcholchai, chairman of the National Legislative Assembly, said the 2014 interim constitution will likely be amended to make charter referendum rules clear and to spell out what would happen if the constitution draft is turned down.
The NLA would discuss the draft charter, not to criticise or commend it, but to increase its chance of being endorsed on the referendum.
"We want the draft to be as good as possible so it will be embraced by the people," he said.
Besides, the interim charter's provision on the referendum should be amended to make it clear, he said.
The issue of a referendum remains controversial even though the 2014 interim charter stipulates one and Prime Minister Prayut Chan-o-cha has stressed that it will be held.
This is because Section 37 of the interim charter says after the draft is finished, it will be put to a referendum and must be endorsed by "the majority of voters", instead of "the majority of voters who cast ballots in the referendum" as written in previous charters.
Lawyers have pointed out that based on the current wording, the draft would never pass. There are 49 million voters in Thailand and if 70% or 34.3 million of them turn out, the draft will pass only if a staggering 24.5 million — 72% of those casting ballots — vote yes, which is unlikely.
In the 2007 charter referendum, voter turnout was just under 60%, and 58.3% of those voting approved the draft.
Concern about the existing definition of "majority" approval was one of the reasons that the former National Reform Council voted down the previous draft written by the Borwornsak Uwanno team in September 2015.
Deputy Prime Minister Wissanu Krea-ngam has dismissed the concern, saying the clause should be interpreted as "half of the voters who come to cast the ballot", even though the wording gives no room for such an interpretation.
He said it could easily be fixed in an amendment if necessary but to date that has not happened.
The interim charter also does not say what will happen if the new draft charter is voted down in the referendum. Some speculate that the junta might pick one of the old constitutions, revise it slightly and use it, but this has never been confirmed.