Mr Pornpetch is favourite for the post while former Senate speaker Surachai Liengboonlertchai is expected to become one of two NLA vice-presidents. Mr Pornpetch is an ombudsman and serves as a legal adviser to the National Council for Peace and Order (NCPO).
NLA member and ex-Uttaradit senator Phirasak Porjit said several former elected senators agreed to support Mr Pornpetch for the role when they met for lunch on Monday.
Mr Pornpetch, who sat on the legal team drafting the provisional constitution, should understand the spirit of the provisional charter, he said.
Mr Phirasak, who is also a potential candidate for one of the vice-presidential posts on the NLA, said he expected the selection of the NLA president and two vice-presidents would go through unopposed.
Somchai Sawaengkarn, an NLA member and former appointed senator, said Mr Pornpetch and Mr Surachai are well-respected legal experts and suited for the posts of NLA president and vice-president.
Mr Somchai denied reports that the former senators had lobbied other assembly members to support Mr Surachai in the race for the presidency.
Mr Surachai said he had no problem working in any position. He said it is up to NLA members to decide who will be president and vice-president of the body.
Srisuwan Janya, activist and secretary-general of the Constitution Protection Association, submitted a petition Monday to the Office of the Ombudsman, asking it to seek a Constitutional Court ruling on whether the establishment of the NLA violates Section 7 of the interim charter.
Mr Srisuwan said the assembly is not represented by a diverse range of professional groups as stipulated by Section 7, with about 115 military and police officers making up more than half of the 200-member NLA. He also asked the court to order the suspension of the NLA's duties until judges deliver their ruling.
Meanwhile, Gen Thawatchai Samutsakhon, a former 2nd army commander, submitted a letter of resignation from the NLA Monday. Gen Thawatchai was on the list of 200 NLA members announced by the NCPO on Friday.
He resigned after it emerged that he was never eligible for the post. Section 8 (1) of the interim charter says an NLA member must not have held any political party post during the past three years.
Gen Thawatchai was a deputy leader of the Chartpattana Party until early this year. He said he was not upset about missing out on the opportunity to sit on the NLA because he can still work for the good of the country. He backed NCPO chief Prayuth Chan-ocha for the post of prime minister.
The Chularatchamontri, Aziz Pitukkumpol, has also declined an NLA seat, saying it conflicts with his role as head of the Muslim religion in Thailand.