Formalising the merger, the JD(U) flag was handed over to Veerendra Kumar by Yadav and former Bihar Chief Minister and party leader Nitish Kumar in front of a huge gathering.
Yadav declared Veerendra Kumar as the President of Kerala JD(U) unit.
He also asked Veerendra Kumar to take efforts to strengthen the party in neighbouring states of Karnataka and Tamil Nadu.
SJD has two MLAs -- K P Mohanan, who is the Agriculture Minister and Shreyams Kumar, son of Veerendra Kumar -- in the 140-member assembly.
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Announcing the merger decision in New Delhi in September last, Yadav and Veerendra Kumar had said they have come together to strengthen "secular democratic forces" as they feel that there is a need to create a platform for this.
Veerendra Kumar was in Janata Dal (Secular) headed by former Prime Minister H D Deve Gowda from 1999 to 2010, when Janata Dal had broken into many factions after some of its leaders decided to join hands with BJP-led NDA at the Centre.