THE leader of Thailand's election-winning Move Forward Party was braced for what could be his final shot at becoming prime minister on Wednesday, as a parliament that denied him last week convened for its second vote on the premiership.

Thailand has been run by a caretaker administration since March and 65 days have passed since Move Forward's stunning election triumph over parties backed by the royalist military after nine years of government controlled by generals.

The 42-year-old, U.S.-educated liberal Pita Limjaroenrat needs the backing of more than half of the bicameral parliament to be endorsed as Thailand's next prime minister, but must overcome fierce resistance from a military at odds with his party's anti-establishment ambitions.

Parliamentary rules written by the military after a 2014 coup and skewed in its favour ensured Pita's defeat in the first round, when he was stopped by a Senate appointed by generals allied with conservatives and old money families that have long wielded influence over Thailand's democracy.

Pita has a mountain to climb and hit his first of a series of expected hurdles on Wednesday when political rivals challenged his re-nomination, arguing parliament had already declined to endorse him.

The House speaker initiated a debate on the issue.

In a post on Twitter as parliament convened, Pita appealed to his rivals to vote according to the will of the people.

"In today's dust and gloom, without knowing what will happen - under this situation - how does one escape the political crisis?" he said.

"In these dark times, holding on to principles and people's opinion is always safe."

'MADE GAINS'

Pita knows that if he fails this time, he must honour his pledge to make way for coalition partner and political heavyweight Pheu Thai to field its prime ministerial candidate in the next round.

In the first vote last week, Pita was 51 votes short and was backed by only 13 of the 249 senators, many of whom abstained or were no-shows, effectively votes against him.

Move Forward believes many were pressured to deny him and Pita is hoping some could change their minds.

"We're still in deficit, but we have made gains," he told Reuters in an interview on Tuesday.

"I reached out to the 13 brave ones - there was a group who didn't show up. They pledged to vote for Pita ... that's a sizable bloc."

In the May election, the progressive Move Forward ran a disruptive campaign, using social media to target urban and young voters with promises of bold institutional reforms to upend the conservative status quo.

But its agenda has put it on a collision course with establishment interests. Besides the challenge to his nomination, Pita faces another obstacle on Wednesday that has raised doubts about whether the parliamentary vote will even take place.

The Constitutional Court is due to consider a complaint against him seeking his disqualification over a shareholding issue deemed in violation of election rules, which could result in his suspension as a lawmaker.

"That's fine," he said of the obstacles. "It's something that was pre-planned."