Panthongtae Shinawatra, who joined Pheu Thai on Sunday, will join senior members in leading the party's political campaign, a party source said on Monday.
The source said Mr Panthongtae will join Khunying Suradat Keyurphan and Chalerm Yubamrung as lead campaigners for the party in the lead up to the general election, tentatively planned for Feb 24.
His presence would boost the morale of party members after seeing key figures defect to other parties, mainly Palang Pracharath, the source said.
Mr Panthongtae's decision also sent a signal to supporters of Pheu Thai that former prime minister Thaksin was still behind the party, the source added.
Mr Pathongtae applied for Pheu Thai membership on Sunday and showed the receipt on his Facebook post. He wrote that he planned a roadshow to woo voters to "restore democracy to the people".
Pheu Thai has seen key members, including the Three Allies led by Suriya Jungroongruangkit and Somsak Thepsuthin, jump ship to Palang Pracharath.
Palang Pracharath supports Gen Prayut Chan-o-cha's return as prime minister after the poll, although it has not officially announced its position.
Pheu Thai opposes an outsider prime minister and has made clear its opposition to the military government.
"The final decision will rest on 40 million ballots on Feb 24," Mr Panthongtae wrote.
Khunying Sudarat, who is in charge of mapping out political strategy for Pheu Thai, travelled to the northeastern region last week to seek support for the party.
The source said she was considering running in a constituency in Bangkok to guarantee a ticket into parliament. If she does, she will pick a constituency where support for the party is strongest, the source added.