Chamlong Srimuang said the PAD and the anti-government protesters who have occupied the Government House will not allow Somchai a grace period to prove himself.
Somchai is elected in the Parliament as prime minister on Wednesday, replacing Samak Sundaravej who was last week disqualified from the job for violating the constitution.
Somchai is brother-in-law of Thaksin who is living in exile in London after jumping bails on corruption charges last month. Thaksin and his family were reportedly seeking political asylum from the British government.
Somchai, a former permanent secretary for justice, is married to Thaksin's younger sister.
"It is useless for Somchai to prove himself with his work as his past deeds were reason enough for PAD to reject him now.
If Somchai wanted the PAD to accept him, then he should try to bring back Thaksin to Thailand to face graft and corruption charges in court," he told reporters.
Chamlong insisted that Somchai should revoke Thaksin's diplomatic passport.