"Sonia Gandhi is likely to share dais with (Bihar Chief Minister) Nitish Kumar and (RJD chief) Lalu Prasad at the rally," a senior leader of the grand alliance said.
He said Samajwadi Party chief Mulayam Singh Yadav may skip the rally.
According to JD-U leaders, after Nitish Kumar had talked with Congress vice president Rahul Gandhi and other senior Congress leader, Sonia Gandhi has agreed to attend the rally.
Nitish Kumar and Lalu Prasad are actively wooing supporters to make their the rally a success.
The event will be their first big show of unity a year after the two leaders joined hands in a bid to defeat the BJP-led National Democratic Alliance (NDA) in the state.
Lalu said the "Swabhiman" rally of the grand alliance would be an answer to "Modi's arrogance and repeated insult to Bihar".
Modi has already addressed three "Parivartan" rallies in Muzaffarpur, Gaya and Saharsa ahead of the assembly polls. He will speak at another rally in Bhagalpur on September 1.