Farooq Naik, a senior lawyer and member of the Senate or the upper house of parliament, said the chairperson of the Pakistan People's Party had received a letter that contained the threat.
The letter was purportedly written by the leader of the suicide squad, Naik told a TV channel. Naik indicated that the bombers might be linked to terrorist groups like al-Qaeda.
The development came five days after a suicide attack on Bhutto's homecoming rally in the southern port city of Karachi claimed 165 lives and injured many more.
Bhutto, who returned to Pakistan on October 18 after eight years in self-exile, had earlier alleged that three senior government officials were behind attempts to assassinate her. She has also claimed that jehadi elements in the establishment, who were close to late military dictator General Zia-ul-Haq, were targeting her.
The government has dismissed her allegations and Interior Minister Aftab Ahmed Khan Sherpao has said that he believes extremists from Pakistan's restive tribal areas bordering
Afghanistan were behind the attack on Bhutto's motorcade.