Move Forward Party (MFP) leader Pita Limjaroenrat's bid for prime minister hangs in the balance as a joint sitting of parliament will convene on July 13 for the crucial vote for a new PM.
House speaker Wan Muhamad Noor Matha said on Wednesday that if a PM candidate fails to win in a first round of voting, further rounds will be held until a candidate secures the required support in the joint sitting.
Meanwhile, it is not clear whether the Pheu Thai Party, which won the second largest number of House seats in the May 14 election, will nominate a candidate and who it will be if Mr Pita fails to win the post.
Key Pheu Thai figures had previously said the party would support Mr Pita's bid but if he failed, Pheu Thai would then consider nominating its own candidate.
It is also not known yet whether the outgoing coalition parties, particularly the Palang Pracharath Party (PPRP) and the United Thai Nation Party (UTN), would also nominate any candidates.
However, speculation has been rife that PPRP leader Prawit Wongsuwon and UTN leader Pirapan Salirathavibhaga may also be nominated if Mr Pita fails to secure the support of senators.
By that time, the PPRP and the UTN would have been expected to lobby hard for support for their respective candidates, sources said.
Mr Wan, also parliament president, said he has scheduled voting for July 13 in consultation with Senate Speaker Pornpetch Wichitcholchai.
The 250 appointed senators are allowed by the constitution to participate in the vote, along with the 500 elected members of the House of Representatives.
Mr Wan said a PM candidate may win the post in the first round of voting on July 13.
If the vote is not in the candidate's favour, another vote will be called.
Under the constitution, a PM candidate would need the support of at least half of the 750 members of both the lower and upper chambers of parliament, or 376 in total.
"Parliament must convene until a prime minister is elected. Mr Pita may not be the only candidate," Mr Wan said.
"Mr Pita could win the post. Otherwise, a new prime minister must be chosen anyway.
"Under the constitution, parliament is duty-bound to elect a prime minister to run the country. The country cannot go without a prime minister.''
Move Forward won the most seats in the general election and has the right to attempt to form a government first.
Mr Wan said the eight coalition parties have 312 votes supporting Mr Pita for prime minister next week.
They need 64 more votes from senators or other MPs to reach the required minimum support of 376.
If Mr Pita is not elected in the first round, the constitution does not stipulate if a previously nominated person or a new candidate should be nominated at the joint sitting.
However, candidates must be from the lists submitted by parties to the Election Commission (EC) and their qualifications must be verified by the poll agency.
If they fail to win endorsement from the House and the Senate, an outsider can be nominated as allowed by Section 272 of the constitution, but the process is rather complicated, Mr Wan said.
Under the constitution, if a joint sitting of parliament fails to select a new premier from party candidate lists, Section 272 would trigger an alternative route.
In such a situation, half of the 750 MPs and senators can initiate a motion to suspend the rule requiring that PM candidates come from party lists, paving the way for an outsider to be selected.
Such a motion requires the support of two-thirds of all the lawmakers, or 500, to suspend the rule.
A nominated outsider would also need at least 376 votes to become prime minister.
Mr Pita faces complaints questioning his eligibility over a shareholding in iTV Plc.
The constitution bars individuals with media shareholdings from running for office.
