Asked if Rahul Gandhi would be accompanying his mother, the PCC chief said he would visit Bihar later for canvassing and hold roadshows.
Sonia Gandhi would share the dais with Bihar Chief Minister Nitish Kumar and Rashtriya Janata Dal chief Lalu Prasad at the rally.
A leader of the alliance said that Samajwadi Party chief Mulayam Singh Yadav might skip the rally.
According to Janata Dal-United leaders, after Nitish Kumar spoke to Congress vice president Rahul Gandhi and other senior Congress leaders, Sonia Gandhi agreed to attend the rally.
Nitish Kumar and Lalu Prasad were actively mobilising supporters to make the rally a success.
The event would 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 in the state.
Lalu Prasad said the Swabhiman rally would be an answer to Prime Minister Narendra Modi’s “arrogance and repeated insult to Bihar”.
Modi has addressed ‘Parivartan’ rallies in Muzaffarpur, Gaya and Saharsa ahead of the assembly polls. He is slated to speak at another rally in Bhagalpur on September 1.