Election victors Move Forward still hope they can renominate party leader Pita Limjaroenrat for prime minister and that senators will give him enough support.
Secretary-general Chaithawat Tulanon said on Monday that the party has petitioned the ombudsman to ask the Constitutional Court if Mr Pita can be renominated to parliament for the position of prime minister at the head of a coalition government.
He hoped the renomination would be allowed and there would be enough support from the military appointed senators.
Mr Pita was nominated as the sole prime ministerial candidate at the joint sitting of the 500-stong House of Representatives and the 249 current senators on July 13, but he failed to win a majority vote. His eight-party alliance mustered only 324 votes, well short of the needed 375.
The parliament then voted on July 19 to reject his renomination. Opponents argued that a failed motion could not be resubmitted during the same session of parliament, and that this also applied to Mr Pita's nomination for prime minister.
After the rejection the Move Forward Party (MFP), which won 151 House seats, agreed to allow the Pheu Thai Party, its largest ally with 141 MPs, to take the lead and nominate a candidate for prime minister. This is expected at a joint sitting on Thursday.
The MFP secretary-general said on Monday that he now believed about 56 senators were ready to support MFP's bid to lead the next government.
He acknowledged that several parties outside the eight-member coalition seemed willing to support Pheu Thai if it excludes MFP from the government. However, Mr Chaithawat said, if the eight-parties remain united there could be a way out.
Leaders of the eight parties were scheduled to meet on Tuesday, he said.
"If Pheu Thai and Move Forward go head and a prime minister is still not elected, society will have many questions for the senators and the other political parties," Mr Chaithawat said.
He rejected the possibility that parties with only a minority vote in the House could form the next government.
MFP secretary-general Chaithawat Tulanon
