No decision has so far been taken on the issue, party spokesman R P N Singh said.
Another leader, who declined to be identified, said that a decision will be taken by tomorrow.
The rally scheduled at Patna's historic Gandhi maidan on August 30 is planned to be the first joint rally by top leaders of the alliance to take on the BJP-led NDA.
Bihar Chief Minister Nitish Kumar and RJD Chief Lalu Prasad are organising the rally in an apparent move to project a united face of the alliance.
He said that the Congress had held district-wise meetings to ensure that the rally becomes a success.