Judgement day for Thailand's embattled Pita Limjaroenrat as parliament votes on PM

The 42-year-old leader of surprise election winners Move Forward is expected to be the only candidate in Thursday's vote.

Thailand's prime ministerial hopeful Pita Limjaroenrat was braced for a critical test of his political clout on Thursday, as parliament convened for a high-stakes vote on the premiership that could test the unity of his eight-party alliance.

The 42-year-old leader of surprise election winners Move Forward was the only candidate nominated on Thursday, but he faces a big challenge in securing the required backing of more than half of the 749-member bicameral parliament in a vote expected by late afternoon.

The liberal Move Forward and its alliance partner, Pheu Thai, thrashed conservative pro-military parties in the May 14 election, seen widely as a resounding rejection of nearly a decade of government led or backed by the royalist military.

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But Pita's determination to pursue Move Forward's progressive, anti-establishment agenda puts him at odds with a powerful nexus of conservatives and old-money families that have loomed large over Thai politics for decades, and will be almost certain to try to thwart him in Thursday's vote.

His alliance controls 312 seats, but to get the required 375 votes, he needs support from some of the 249 members of the conservative-leaning upper house Senate, which was appointed by the military after a 2014 coup.

One senator resigned on Wednesday, lowering the threshold to 375 votes to become prime minister.

"I'm confident that I will do my best to match the hopes and encouragement from the people," Pita told reporters.

"I will do my best to explain to those senators who still have questions. I'll use this opportunity to find a consensus."

Big Blow

Pita has had a bumpy ride and was dealt a major blow on the eve of the vote when two legal complaints against him gained momentum, prompting hundreds of pro-democracy demonstrators to gather in Bangkok to warn of moves afoot to keep Move Forward from power.

Political uncertainty has pulled Thailand's main stock index down about 11 per cent so far this year. It was little changed in early trade on Thursday, while the baht strengthened, hovering near a six-week high against the dollar.

The Constitutional Court agreed on Wednesday to take on a complaint against the party over its plan to amend a strict law that prohibits insulting the monarchy, just hours after the election commission recommended the court disqualify Pita as a lawmaker over a shareholding violation.

"There have been attempts to block, not to block me but block the majority government of the people from getting to run the country in various ways," Pita told ThaiRath TV on Thursday.

"This is quite normal for the path to power in our country... I am encouraged and hopeful to fix things as they come until the dream of mine and the people can be reached."

The cases against Move Forward are the latest twist in a two-decade struggle for power in Southeast Asia's second-biggest economy fraught with coups, court interventions and crippling, at times violent street protests.

More turbulence can be expected if Pita cannot prevail in the vote given Move Forward's massive support from young voters and its popularity in the capital.

The alliance has had disagreements in recent weeks and if Pita fails on Thursday, it must decide whether to back him again in another vote slated for July 19, or put forward another candidate, testing its cohesion as it seeks to form the next government.