File picture of jailed MLA Mukhtar Ansari.
Mukhtar Ansari, who heads the Quami Ekta Dal and is in prison, has withdrawn as a candidate in Varanasi, the Uttar Pradesh constituency from where Narendra Modi, the BJP's prime ministerial candidate, is contesting the general election.
Mr Ansari's brother Afzal Ansari said he was doing so "to strengthen secular forces and avoid division of votes".
"Now SP, BSP and AAP have to decide how to defeat Modi," Afzal Ansari said.
While announcing that its local leader and legislator Ajai Rai would take on Mr Modi in Varanasi, the Congress had on April 9 appealed to other parties to join ranks and help it defeat the BJP leader, seen as a frontrunner for the top post.
Mukhtar Ansari is currently lodged in the Agra jail in connection with a murder case. He had contested the 2009 general elections as the Bahujan Samaj Party or BSP candidate and had lost by a small margin to the BJP's Murli Manohar Joshi.
Mr Rai, contesting that year as the Samjawadi Party's candidate had come third.
Mr Ansari was part of the Mayawati-led BSP till April 2010, after which he formed the Quami Ekta Dal.
The Varanasi contest is being keenly watched after Aam Aadmi Party chief Arvind Kejriwal announced that he would challenge Mr Modi in the temple town considered a BJP stronghold.