By Chayut Setboonsarng and Panarat Thepgumpanat
BANGKOK (Reuters) -Thailand's Move Forward party filed a motion in parliament on Friday seeking to curb the power of the military-appointed Senate, a day after the body thwarted its party leader's bid to become prime minister.
The role of the 249-member Senate in deciding a prime minister along with the elected lower house - a system designed by the royalist military after a 2014 coup - is seen as a constitutional safeguard to protect the interests of the generals and the conservative establishment.
Move Forward won the most seats in an election in May but despite being unopposed and having the backing of his eight-party alliance, its leader Pita Limjaroenrat lost the crucial vote on the premiership on Thursday, after the Senate and parties of the outgoing, army-backed government closed ranks to deny him the top job.
Only 13 senators backed 42-year-old Pita, with the rest voting against him or abstaining, which his party said indicated some were acting under duress.
Party secretary general Chaithawat Tulathon filed a motion on Friday to amend part of the constitution, saying "This is a solution that all sides will feel comfortable with".
"There are forces from the old power to pressure the Senate - from the old power to some capitalists who do not want to see a Move Forward government," he said in an earlier television interview, adding it could take about one month to pass.
Pita, a liberal from the private sector, has won huge youth support for his plan to shake up politics and bring reforms to sectors and institutions long considered untouchable.
That includes the monarchy, more specifically, a law that prohibits insulting it, by far Move Forward's most contentious policy and a big obstacle in its attempts to persuade legislators to back Pita.
MAJOR BLOW
Pita vowed on Thursday not to abandon those policies or give up his fight for the premiership. He can run again if nominated in the next vote for the post, which takes place on July 19, the House speaker confirmed.
The defeat on Thursday followed a major blow for Pita on the eve of the vote, when the election commission recommended he be disqualified over a shareholding issue, followed hours later by the Constitutional Court announcing it had taken on a complaint over his party's plan to amend the royal insult law.
The political tension this week had been widely expected.
Thailand has been locked for two decades in a power struggle between reform-minded parties that win elections and a nexus of old money and the military establishment determined to stifle them.
Pro-democracy groups have called for protests. Activist group the United Front of Thammasat and Demonstration took aim at the senators and those who abstained in the vote, calling them spineless and "toxic to the will of people".
Thitinan Pongsudhirak, a political science professor at Chulalongkorn University, called the constitution a straitjacket on democracy, and said systematic attempts to stop Move Forward would see a public backlash.
"These old guard institutions, they need to maintain power because they have a lot to lose," he said.
"The kind of change that Move Forward demands would unwind Thailand's monarchy-centred system and then it would unlock institutional reforms... this would unleash a lot of the competitiveness of Thailand, Thailand's potential."
(Additional reporting by Napat Wesshasarter and Juarawee KittisilpaWriting by Martin PettyEditing by Frances Kerry)
