Sonia Gandhi had last month urged parties to set aside minor differences.
The Congress' Sonia Gandhi hosts dinner tonight for leaders of 17 opposition parties at her 10 Janpath home in Delhi. Dinner conversation is expected to revolve around uniting against the BJP ahead of the 2019 national election when Prime Minister Narendra Modi will seek a second term.
Sonia Gandhi, who is chairperson of the Congress-led national alliance UPA, had last month urged parties to set aside minor differences to get together in "the larger interest of keeping the BJP out of power," stating that she will work with like-minded political parties to defeat the BJP in the 2019 Lok Sabha election.
Trinamool Congress chief and West Bengal chief minister Mamata Banerjee has deputed her party's senior leader Sudip Bandhopadhyay to attend. Samajwadi Party chief Akhilesh Yadav is expected to send his uncle Ram Gopal Yadav.
Bahujan Samaj Party chief Mayawati has been invited, but is unlikely to attend or even send a representative.
Sources said contrary to some reports, Andhra Pradesh's Telugu Desam Party, which recently pulled out its ministers from PM Modi's government but remains a part of the ruling alliance NDA, has not been invited. Neither have Naveen Patnaik's Biju Janata Dal, Odisha's ruling party, or the Telangana Rashtra Samithi (TRS) which is in power in Telangana.
TRS boss and Telangana chief minister K Chandrashekhar Rao has just announced a new mission to float a non-BJP, non-Congress third front, seeking to tap some parties attending Mrs Gandhi's dinner today.
Mamata Banerjee greeted Mr Rai's suggestion with much enthusiasm, declaring her complete support and saying, "I will work with you." The Jharkhand Mukti Morcha's Hemand Soren also welcomed Mr Rao's initiative.
Mr Soren is expected to attend today's dinner as is Babulal Marandi of the Jharkhand Vikas Morcha. From Bihar, jailed RJD leader Lalu Yadav's son Tejashwi Yadav is expected to attend, but there was no immediate confirmation. The Left parties will be there.
The dinner, which sources described as "customary", comes amid renewed push by opposition parties to take on the Narendra Modi government in parliament on several issues including the Rs 12,600-crore Punjab National Bank (PNB) fraud case. A united opposition has disrupted parliament for over a week now, ever since it convened for the rest of the budget session after a recess.
At other dinners hosted by Mrs Gandhi for these 17 other opposition parties, a united approach to the presidential and vice presidential elections last year were discussed. Sonia Gandhi stepped down as Congress president last year after an uninterrupted run of 19 years. Her son Rahul Gandhi is now Congress president.