Future Forward leader Thanathorn Juangroongruangkit called for change on Wednesday, as both houses of parliament convened for the first time since a 2014 coup to vote for either him or the chief of the ruling junta to be leader.
The vote will officially restore civilian rule after more than five years under junta leader Prayut Chan-o-cha, who has been nominated to stay on as prime minister by a coalition led by the pro-army Palang Pracharat Party.
The retired general is considered to have a big advantage over Mr Thanathorn, a charismatic political newcomer nominated by the opposition Democratic Front of seven parties that want to remove the military from politics.
"Coups will only lead Thailand to dead-ends ... We must make parliament a place of honour, not a place where people's faith goes to die," Mr Thanathorn said in a speech just outside the parliamentary chamber.
"I am ready to be Thailand's next prime minister ... I will be the prime minister of change."
Mr Thanathorn was barred from taking up his seat in the lower house last month after being suspended, pending a ruling by the Constitutional Court on his alleged illegal holding of shares in a media company.
He denied the allegation and said the case was the latest politically motivated bid to block him from politics.
The vote for a prime minister who will form the next government this month, comes 10 weeks after a March 24 general election the opposition says was heavily weighted to favour pro-army parties.
Each parliamentarian will be called on by the house speaker to verbally vote for either candidate. It could be hours before they start voting.
ABHISIT OUT
A new prime minister will need at least 376 votes, or more than half the parliament. That includes 500 members in the lower House of Representatives elected in the March election and 250 senators, who were all appointed by the junta.
Neither the opposition nor Palang Pracharath’s side officially has a majority in the lower house, with 246 seats and 203 respectively.
Palang Pracharath gained the Democrat Party as a new ally on Tuesday and that would likely also attract the Bhumjaithai Party, which holds 51 seats but hasn't officially declared an alignment with either side.
Former Democrat Party leader Abhisit Vejjajiva, who is also a former prime minister, gave up his seat in parliament earlier on Wednesday, saying he could not break a pre-election promise and vote along with his party to support Gen Prayut.
"We have seen abuse of state power ... The (junta's) extension of power is not just rhetoric, it is reality," Mr Abhisit said.
The Democratic Front has said the appointed Senate's votes give Gen Prayut an unfair advantage and urged senators to vote in line with the will of the people.
With the backing of the Senate, Gen Prayut could easily gain enough votes to stay in power.
The junta will remain in power until a new cabinet is sworn in.