The Pheu Thai Party is seeking a legal channel to challenge the senator selection process, saying the selection committee's members were not politically neutral.
Pheu Thai legal expert Chousak Sirinil said it is questionable whether the formation of a nine-member panel responsible for selecting Upper House members is constitutional. "In my opinion, it is not," he said.
The charter stipulates the panel must consist of experts from various fields and be drawn among those who are politically neutral, he said.
The names show they were allied to the National Council for Peace and Order (NCPO), which is not politically neutral, he added.
The panel-appointed senators, along with MPs, played a part in choosing the premier while one of the prime ministerial candidates was also head of the NCPO, he said, adding: "This is a conflict of interest."
"This shows the selection of senators ran afoul of the constitution," Mr Chousak said.
He said Pheu Thai is exploring legal channels through which to lodge a petition challenging the selection process.
A petition could be lodged with the Constitutional Court to rule on the matter, according to Mr Chousak.
The names of the original 10 member Senate-selection panel were revealed by the NCPO on Wednesday. Six were found to be among those eventually appointed senators.
The panel was originally made up of 10 members, but former National Legislative Assembly president, Pornpetch Wichitcholchai, later quit. Mr Pornpetch is now the Senate speaker.
Mr Pornpetch said he quit the panel because he was too busy as National Legislative Assembly speaker.
He said as far as he was aware, when the names of the panelists were raised as potential senators, they had to leave the meeting room.
He said he was not worried about opposition parties scrutinising Upper House decisions more closely, saying senators have to perform their duties in line with laws and the charter.
Responding to criticism as to why the names of the selection panelists were not revealed in the Royal Gazette in the first place, Deputy Prime Minister Wissanu Krea-ngam said it was an internal matter for the NCPO and did not concern the public.
Meanwhile, activist Srisuwan Janya is to file an Election Commission petition today against several MPs for breaching media shareholding rules.