BANGKOK, July 14 (Reuters) - Thailand's election winning Move Forward party announced on Friday a plan to try to curb the power of the military-appointed Senate, a day after its members thwarted the 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.

Despite being unopposed and with the backing of his eight-party alliance, Move Forward's 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.

"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," party secretary general Chaithawat Tulathon said in a television interview.

"Since the senators were uncomfortable in electing the PM, why not switch off this power?" he said, adding the party would attempt to limit the Senate's powers by amending an article of the constitution, which could take a month.

Pita, a Harvard-educated 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 convince legislators to back Pita.