Ram Kripal Yadav was upset over RJD chief Lalu Prasad's daughter Misa being made the candidate from Patliputra constituency in Bihar.
He joined the Bharatiya Janata Party (BJP) in presence of party president Rajnath Singh.
"I have been a dedicated worker of the RJD for 35 years but was forced to take this step," said Yadav after joining the BJP.
"In RJD, family is getting preference over others. I was pained at what was happening to ordinary workers like us. We felt that social justice was what we must fight for and not for one family," he said in an attack on Lalu Prasad, whom he was once close to.
Yadav added that he will work as a grass root level worker for the BJP.