An eight-party alliance seeking to form Thailand's next government proposed Move Forward leader Pita Limjaroenrat as a candidate to contest a prime ministerial vote in parliament on Wednesday.

If his nomination is approved, it would be his second attempt to win the endorsement of the bicameral legislature after losing in his first attempt last week.

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, US-educated liberal Pita 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 ensured his 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 in Thailand.

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 then 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."

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 fell 51 votes short, and was backed by only 13 of the 249 senators, many of whom abstained or were no-shows, effectively voting against him. (Reuters)