Varawut Silpa-archa, chairman of Chartthaipattana Party's policy and strategy committee, said he will personally vote for the charter amendment bill in the third and final reading on Friday.
Mr Varawut, who is minister of natural resources and environment in the Prayut Chan-o-cha government, said his party's MPs would meet on Wednesday morning to discuss the matter.
He would personally vote for the bill, which seeks the return of the two-ballot election system - in line with the 1997 constitution backed by his late father and former prime minister Banharn Silpa-archa.
However, the party's MPs would be allowed a free vote, he said.
Mr Varawut said the one- and two-ballot election systems both have strong and weak points. He believed some senators would hold a different opinion from him.
Sen Wanchai Sornsiri on Monday said the proposed return of the two-ballot system would be more a step backward than a step forward. He said the senators' position should be clear one day ahead of the scheduled vote on the bill on Friday.
