Two senators have indicated they will not vote to support Move Forward Party leader Pita Limjaroenrat as the new prime minister, questioning his attitude towards the monarchy.
Senator Jadet Insawang said on Monday that he took oath on assuming office that he would protect the present constitution.
Sections 6 and 50 of the constitution required everyone to protect the constitutional monarchy, he said.
The Move Forward Party and Mr Pita had previously announced their intention to revoke Section 112, which related to His Majesty the King. That would lead to discrediting the royal institution, and was unacceptable, Mr Jadet said.
Section 112 is the lese majeste law under the Criminal Code.
"If Mr Pita is nominated [for prime minister], I will reject it because I will observe the constitution and keep my oath," he said.
"The institution has nothing to do with you. You will change Section 112 and lessen the penalty. I am asking why," Mr Jadet said.
Senator Kittisak Ratanawaraha said a candidate for prime minister must be loyal to the nation, its religions and the monarch.
Senators would consider the possible impact on the nation before making their decisions, and they had proved to be mostly unanimous in their past votes, he said.
The senator also said that the Election Commission had yet to consider a complaint against Mr Pita's qualification for the position.