The 44 facing trial from June 30 for alleged ethics violations include serving and former MPs of the People's Party and its disbanded predecessor Move Forward, who each face the maximum penalty of a lifetime ban from office if found guilty.
Hundreds of people have been prosecuted in recent years under Thailand's lese-majeste law, which is among the strictest of its kind in the world, with punishments of up to 15 years in prison for offenders.
On the back of massive anti-government, youth-led street protests that included calls for the reform of the monarchy, Move Forward tried to amend the law in parliament in 2021, arguing it had been misused to stifle political opponents.
Thailand's constitution enshrines the king in a position of "revered worship" and royalists regard the palace as sacrosanct.
The liberal agenda of the People's Party and its forerunners created powerful enemies among Thailand's conservative establishment, with the movement on the wrong end of a multiple court rulings, including two party dissolutions.
The 44 include People's Party leader Natthaphong Ruengpanyawut, four deputy leaders and former Move Forward leader Pita Limjaroenrat, who is currently serving a 10-year ban from politics.
"We will pursue our legal fight in the Supreme Court to the fullest extent, in order to defend the legitimacy of the representative mandate in a parliamentary democracy," People's Party leader Natthaphong told a media conference on Friday.
The progressive movement's hugely popularity among young and urban Thais swept Move Forward to victory in the 2023 general election, but it was blocked from forming a government by MPs allied with the royalist military.
A court in 2024 ruled that Move Forward's attempt to reform the royal insult law was unconstitutional and undermined the democratic system.
It was later dissolved by the same court and its top leaders banned, but members regrouped days later as the People's Party.
The Supreme Court case stems from a petition lodged by Thailand's National Anti-Corruption Commission, which has a wide investigative remit beyond graft.
The court said serving MPs among the 44 implicated would not be suspended.
Despite big leads in opinion polls, the People's Party was the surprise runner-up in February's general election to Prime Minister Anutin Chanvirakul's Bhumjaithai Party.
