Unofficial returns from Sunday's election gave the PPP 232 of the 480 seats in parliament, just short of the absolute majority needed to govern alone.
Party leaders have been trying to build a ruling coalition with smaller parties and have confidently predicted they will be forming the next government.
"PPP needs to secure at least 280 seats to make a strong coalition government," said Ukrist Pathmanand, a professor of political science at Bangkok's Chulalongkorn University.
The Democrat Party, which came in second with 165 seats, has already refused to join a PPP-led government. The remaining 83 seats were split among five smaller parties.
On Monday, PPP secretary general Surapong Suebwonglee said the party had enough support to form a coalition government, but he refused to reveal their partners.
However quoting a PPP source, the English-daily Bangkok Post said Tuesday the pro-Thaksin party had "successfully talked" three parties - Chart Thai, Puea Pandin and Matchima Party - "into helping it form a government."
The Nation also said PPP had secured support from at least two small parties - Matchima Party and Pracharaj.
