Trinamool Congress leader Sukhendu Shekhar is likely to be elected as the opposition candidate for the post of Rajya Sabha deputy chairperson, which is set for a contest during the monsoon session of Parliament starting from July 18.
The Congress has decided to back Mamata Banerjee's candidate despite having numbers to claim the post is seen as a step ahead towards stitching an anti-BJP front ahead of 2019 general elections.
The opposition's combined strength in the upper house including the erstwhile BJP-ally TDP has 117 votes. But the winning candidate will require 122 votes in the 245-member house.
The BJP, which is the single largest party in the Upper House has the support of 106 members, including that of 14 AIADMK members.
The other two parties - TRS with 6 members and the YSRCP with two members - are yet to open their cards on this crucial election which is seen as a test of the opposition's unity.
Odisha Chief Minister Naveen Patnaik's Biju Janata Dal (BJD) and Telangana Rashtra Samiti (TRS) are also backing Mamta's candidate and in such a situation Congress would have to back him as well to ensure the victory of a non-BJP candidate.
The election for the post of Rajya Sabha deputy chairperson is happening after 26 years.
Last time when the election for deputy chairperson took place, Congress candidate Najma Heptullah, now a BJP leader, was up against Opposition's Renuka Chowdhary, who is now with the Congress.
The post is currently held by PJ Kurien, Congress MP from Kerala who was denied another term to the Upper House by his party.