With the 'who's who' of anti-Bharatiya Janata Party politics expected to be present, considerable significance is being attached to the get-together ahead of the Monsoon Session, which is likely to be the most turbulent for the Narendra Modi government since it took office in May last year.
Among the leaders invited are Mulayam Singh Yadav (Samajwadi Party), Mayawati (Bahujan Samaj Party), Sharad Pawar (Nationalist Congress Party), Sitaram Yechury (Communist Party of India-Marxist), H D Deve Gowda (Janata Dal-Secular), E Ahamed (Indian Union Muslim League), Kanimozhi (Dravida Munnetra Kazhagam), D Raja (Communist Party of India), Farooq Abdullah of the National Conference and Sudip Bandhopadhyay (Trinamool Congress).
Though Rashtriya Janata Dal chief Lalu Prasad has been invited, he has expressed inability to attend due to prior commitments. Bihar Chief Minister Nitish Kumar and Janata Dal-United president Sharad Yadav are among the invitees.
The All India Majlis-e-Ittehadul-Muslimeen, which was part of the Congress-led United Progressive Alliance until two years back, and AIUDF, which has a tie-up with the Congress in Assam, are also invited.
The regional parties which are not part of the BJP-led National Democratic Alliance, but have still been kept away, include the AIADMK, Biju Janata Dal and Telangana Rashtra Samithi, which rule Tamil Nadu, Odisha and Telangana respectively.
Congress' ties with these parties within and outside Parliament are at best lukewarm.
In Bihar, the JD-U, the RJD, the Congress and the NCP have resolved to fight unitedly under the leadership of Chief Minister Nitish Kumar, while the left will go to the hustings as a separate bloc.
Congress is utilising the opportunity to send a message of larger opposition unity at a time when the BJP and the Modi government are on the backfoot following a series of controversies after revelations of External Affairs Minister Sushma Swaraj and Rajasthan Chief Minister Vasundhara Raje extending help to "fugitive" former Indian Premier League boss Lalit Modi.
It is also facing heat for the Vyapam scam in which the Supreme Court has ordered a CBI probe.
The get-together comes at a time when government managers have decided to brazen it out both outside and within Parliament, reflected in the decision of not shortening the monsoon session.
With the Modi government and BJP rejecting demands for resignation of Swaraj, Raje and MP Chief Minister Shivraj Singh Chouhan, fireworks are expected during the session which will conclude on August 13.
Congress has also been demanding the resignation HRD Minister Smriti Irani for misrepresenting facts about her educational qualifications, and Maharashtra minister Pankaja Munde for alleged involvement in a "scam" by clearing purchases worth Rs 206 crore on a single day without inviting tenders in violation of norms.
Chhattisgarh Chief Minister Raman Singh too is in the line of Congress fire over a "rice scam" wherein Rs 36,000 crore was alleged to have been misappropriated.
Image: Congress President Sonia Gandhi with RJD chief Lalu Yadav, JD(U) chief Sharad Yadav and NCP leader Tariq Anwar (R) during her Iftar party in 2014.