Puea Thai Party, seen as a spare of disbanded People Power Party, is scheduled on Sunday to elect its leader and 29-member executive board.
The naming of the party leader is seen as the crucial step for the nomination of the next prime minister to be voted in the House, tentatively scheduled for Monday.
On Wednesday, about 80 of 218 MPs from the disbanded ruling party have already completed the process to switch to the Puea Thai banner.
Deputy House Speaker Apiwan Wiriyachai, seen as a contender for premiership, is now a full-fledged Puea Thai member.
Emerged from the registration of his party membership, he said he viewed Industry Minister Mingkwan Saengsuwan as a more suitable candidate for the position of prime minister.