However, a day of intense political bargaining saw most of those MPs agree to back Somchai as the party's nominee.
Somchai was named as Thailand's caretaker prime minister last week, after Samak was forced to stand down over his hosting of a cooking show while in office.
Somchai had earlier said that dissolution of the Thai parliament and the calling of fresh elections was an option if the ruling party could not reach agreement.
"I will do my best to resolve the ongoing problems," he told reporters in a Tuesday night news conference, flanked by representatives of the other five parties in the governing coalition.
Nonetheless, Somchai remains firmly opposed by the People's Alliance for Democracy (PAD) – a pressure group of royalists, businessmen and union activists – which has been leading weeks of sometimes bloody anti-government protests.
The PAD, which is strongly opposed to Thaksin and anyone seen as associated with him, has vowed to continue with its protests if Somchai is made prime minister.
PAD leaders say Thaksin, who recently fled into exile in the UK, bought his way to power and enriched himself at the country's expense.