Prayut, who seized power in 2014, is all but assured of completing his transformation from junta head to civilian leader with the support of 250 hand-picked senators.
But he faces competition from Thanathorn Juangroongruangkit, leader of the Future Forward Party, which scooped up millions of votes in a March poll mainly from the kingdom's millennials.
In an unexpected move late Tuesday, Thanthorn was announced as the sole prime ministerial candidate of the anti-junta coalition, despite being enmeshed in legal woes that could see him banned from politics and jailed.
Thailand remains bitterly divided after 13 years defined by coups, violent street protests and short-lived civilian governments.
At root is a rivalry between an arch-royalist conservative establishment -- buttressed by the courts and the army -- and pro-democracy parties supported by many of the lower and middle class.
Parliament is deciding on a premier more than two months after the first election since the last coup, a poll marred by allegations of inaccurate counting and vote-buying.
Prayut's allies say he is a stabilising figure who can steer Thailand away from its perennial political crises.
"Prayut has the qualities, has the capability, the leadership to do the job," Uttama Savanayana, the leader of junta proxy party Palang Pracharat, told AFP.
But his critics say the famously gruff ex-army chief represents a narrow elite and lacks the vision or temperament to govern as a civilian leader.
- Unexpected third force -
The election was cast as a choice between junta-backed rule and parties aligned with former premier Thaksin Shinawatra and his sister Yingluck, whose administration was toppled in 2014.
But an unexpected third force emerged led by billionaire auto-parts scion Thanathorn.
His Future Forward won more than six million votes and 81 seats to become Thailand's third largest party.
The social media savvy 40-year-old is now heading a coalition with the Shinawatra's main party -- Pheu Thai -- and five others.
Analysts say Thanathorn represents the greatest challenge to the junta and its establishment allies, with his articulate calls to bridge the kingdom's chasmic social inequality and end the military's influence over politics.
But Thanathorn is weighed down by legal problems.
Those led to his suspension from parliament and a dramatic walk-out from its early sessions.
He said Tuesday that the suspension had nothing to do with qualifying as a candidate for prime minister, and he called on swing parties to back him in the vote.
"The most important thing is to return Thailand to democracy," he told reporters. "And to stop Prayut to come back as prime minister."
Assuming the Thai senate votes with the junta, Thanathorn would need a gargantuan 376 votes from the lower house to emerge as premier.
Prayut, who may not attend the vote, only needs 126.
He received a further boost on the eve of the vote with Thailand's oldest party -- the Democrats -- throwing their lot in with him.
That should smooth his path to office, but analysts say troubles lie ahead for a military man unused to debate and consensus-building.
"Politicians will try to make scenes... to expose Prayut," said Titipol Phakdeewanich, a political scientist as Ubon Ratchathani University.