However, respected social critic Prawase Wasi has agreed to join the reform push.
Ex-premier Banharn Silpa-archa, who is also the reform assembly co-ordinator, met Mr Sondhi and Maj Gen Chamlong yesterday at Ban Phra Athit residence in Phra Nakhon district to invite the former PAD leaders to join the initiative.
Mr Banharn was accompanied by key Chartthaipattana Party member Somsak Prisnananthakul and businessman Sawat Horungruang.
The discussion was broadcast on the ASTV satellite television channel.
Mr Banharn urged the two former yellow-shirt leaders to take part in the political reform assembly, saying it would help strengthen democracy and promote good governance and trust among the public.
Turning down the invitation, Maj Gen Chamlong said the country's conflicts have been caused by the government itself due to its push for the amnesty law, the 2.2-trillion-baht infrastructure loan bill and constitutional amendments.
There is no need for a reform assembly as the government can solve the conflict at its roots simply by ceasing to cause it, he said.
Mr Sondhi said he sympathised with Mr Banharn for having been asked to help the government cool down the political climate.
He raised many questions for Mr Banharn to answer, such as whether he thinks former prime minister Thaksin Shinawatra should return to the country and serve his jail term handed down by the Supreme Court, and whether he agreed with passing a law to grant an amnesty for Thaksin.
"If they use you [Mr Banharn] as a pawn in their game, I'm not in," Mr Sondhi said. "And I want you to think whether you are also the cause of all the problems or not."
Mr Sondhi said he wanted to see Mr Banharn do "something big" to prove he is not Thaksin's pawn.
"I have never thought Thaksin wants political reform. He just wants to cool down the political climate," he told Mr Banharn.
Mr Banharn said he was sincere in seeking their cooperation in political reform and denied being anyone's pawn. He also hailed Prime Minister Yingluck Shinawatra's sincerity in the reform push.
He said the proposed political amnesty would not aim to help Thaksin.
At the end of the talks, Mr Banharn presented souvenir pens specially made for his birthday, to the former PAD leaders.
In the afternoon, Mr Banharn met Dr Prawase at the National Health Foundation in Chatuchak district.
Dr Prawase told him he was ready to support the reform plan.
"Reform is necessary for Thailand since the country has faced difficulties in pushing for reform in various aspects for a long time," he said.
He suggested setting up three pillars to facilitate reform _ a national reform committee chaired by the prime minister, a civic reform network and a reform assembly.