The anti-military alliance will on Wednesday nominate Future Forward Party leader Thanathorn Juangroongruangkit as prime minister in a face-off against Prayut Chan-o-cha, the Future Forward Party (FFP) spokeswoman said.
The scheduled vote for a new prime minister comes five years after then-army chief Gen Prayut seized power from an elected government in 2014.
Mr Thanathorn entered politics just last year and his party came in third in elections in March.
"The seven democratic parties coalition have jointly decided to nominate Thanathorn for the prime minister's vote tomorrow," FFP spokesman Pannika Wanich said.
Pheu Thai Party secretary-general Phumtham Wechayachai said the party agreed to support Mr Thanathorn to be the candidate for the seven-party alliance. Pheu Thai will not nominate a candidate to contest the Future Forward Party on Wednesday, he added.
Pheu Thai's decision followed the position of Khunying Sudarat Keyuraphan, who confirmed her intention not to take a political position.
The pro-regime camp, led by the Palang Pracharath Party, will propose Gen Prayut to continue his premiership in the post-election government.
Mr Thanathorn said he has prepared to deliver a vision speech because people should know how the next premier will run the country.
“If the House gives its permission, I am ready to give my vision. I will stand by whether or not Gen Prayut is present,” he said.
Mr Thanathorn took an opportunity to thank Pheu Thai for showing its spirit. “It won the highest number of votes in the March 24 election, but it has made a huge sacrifice in renouncing all privileges. I also thank other anti-regime parties for pledging allegiance to democracy,” he said.
