Senators now expect Pheu Thai Party's prime ministerial candidate Srettha Thavisin to respond in parliament to questions about his ethical standards following various accusations made against him.
They want to hear from him before they decide whether to support him in tomorrow's prime ministerial vote. Mr Sretta has previously expressed some reluctance to show, as he is not an elected member of the House or Senate.
Pheu Thai needs about 60 votes from the 249-member Senate for its prime ministerial candidate to win the parliamentary vote and become Thailand's next prime minister.
In the case the candidate nominated for Tuesday's vote fails to muster enough support from the Senate, that candidate cannot be re-nominated, in line with the precedent set on July 19, when the nomination of Move Forward Party (MFP) leader Pita Limjaroenrat for a second vote was rejected.
Senator Somchai Sawangkarn on Sunday said he wanted a clear answer from Pheu Thai as to which of its three prime ministerial candidates will actually be nominated.
He was responding to rumours that the party was considering swapping Mr Srettha with either Paetongtarn Shinawatra or Chaikasem Nitisiri, after accusations by whistleblower Chuvit Kamolvisit against Mr Srettha raised questions over his business dealings.
If Pheu Thai wants to switch to a new candidate, the Senate should be informed in advance so it will have enough time to prepare to scrutinise the candidate's qualification before the vote takes place, said Mr Somchai.
"If senators are left with insufficient time to examine the new candidate's qualification, they might not vote for him or her," he said.
"If Pheu Thai insists on nominating Mr Srettha, he should come to Tuesday's joint sitting of the House of Representatives and the Senate to deliver a statement demonstrating his vision as a would-be prime minister, and respond to questions which may be asked before the vote," Mr Somchai said.
Mr Srettha's response to Mr Chuvit's accusations was not enough to address the questions the public may have about the alleged misconduct, said Mr Somchai.
He added that many senators who had already promised to throw their weight behind the party are now doubting their position because of the accusations made by Mr Chuvit.
He was referring to alleged tax evasion carried out by Sansiri Plc, the property development company for which Mr Srettha had served as CEO prior to throwing his hat in the political ring.
"The chances [of Mr Srettha winning the support of the senators] stand at fifty-fifty.
"It will largely depend on how the party handles the situation in the days to come," he said.
Sen Seree Suwanpanont said if Mr Srettha doesn't show up to answer questions in person, the senators will have to base their decision only what Mr Srettha's representative has to say.
In addition to questions surrounding Mr Srettha's ethics, a number of senators are eager to ask the Pheu Thai's candidate about the party's plan to implement its 10,000-baht digital wallet policy, to ensure the policy won't hurt the country's finances, he said.
Meanwhile, Sen Direkrit Jenklongtham said he believed most MPs and senators will base their decision on two factors -- the candidate's qualifications and whether or not the candidate's party has the support of the lower House.
Aside from these two factors, the Senate will also take into consideration the candidate's ethical and moral standards, along with his or her vision on how to implement the government's core policies, said Mr Direkrit.
With regards to Mr Srettha's vow to rewrite the constitution immediately after the government takes office, the senator said he will have to explain why the charter needs to be rewritten in such a rush, and what benefits the amendment would bring the public.
He encouraged Mr Srettha to attend Tuesday's meeting, calling it a good opportunity for him to convince MPs and senators that he is suited to be the next prime minister.
Meanwhile, Ms Paetongtarn said Pheu Thai still hopes to see Mr Srettha win the forthcoming vote.