"It is going to be a historic rally. A large crowd will attend the rally. It is a prestige issue for us," Delhi Congress spokesperson Mukesh Sharma said.
In the run-up to the Delhi assembly elections, in which the party was routed, a rally of Congress vice president Rahul Gandhi witnessed a very low turnout.
The failure of his rally ruffled the feathers of the top brass, and sources said the party will make sure this is not repeated now.
Sonia Gandhi's rally will take place Sunday at Ajmal Khan Park in Karol Bagh in the New Delhi parliamentary constituency represented by Ajay Maken.
Sharma also said Wednesday's rally of BJP's prime ministerial candidate Narendra Modi in the capital was a "flop show".
He said 35,000 to 40,000 Congress workers will be present at the rally.