"I had parted ways with him on differences over certain issues," Kumar said at an election rally addressed jointly with the RJD supremo, the second such campaign by them for bypolls in 10 assembly seats in Bihar to be held on August 21.
Prasad was seated on the dais when Kumar spoke about his parting ways with the RJD chief.
The senior JD(U) leader said this time he has joined forces with Prasad to keep the resurgent BJP away from power in Bihar during 2015 assembly polls.
Charging the Modi government at the Centre with being arrogant, he said the grand alliance of the RJD, JD(U) and Congress will give "medicine" to BJP from Bihar.
Prasad rued that the secular forces lost the general elections due to division of votes, but said no such mistake will be committed this time as the three major parties - RJD, JD(U) and Congress - have come together to stop BJP's juggernaut.
"BJP won due to division of votes of secular parties ... We polled 1.60 crore votes in the general elections and BJP less than us (in Bihar), but yet it walked away with lion's share of seats (31 out of 40) in partnership with LJP and RLSP," the RJD supremo said.
"This time we will put BJP in its place after stitching together votes of RJD, JD(U) and Congress," he said.
Attacking LJP supremo and Union Minister Ram Vilas Paswan, Prasad said he has not seen a "weather scientist" like him, accusing him of "changing tack to suit his own political requirements without carrying a baggage of ideology".
"He is a rank opportunist who switches sides at the drop of a hat," the RJD supremo said.
He reminded Paswan that he had resigned from the Vajpayee government over the Godhra riots in 2002, but has now conveniently joined hands with BJP and was serving as minister in the government headed by Narendra Modi, who was Gujarat Chief Minister then.
Senior Congress leader C P Joshi also spoke on the occasion and charged BJP with "cheating" people by failing to bring about promised development to them.
Earlier, the leaders of the RJD-led alliance had addressed an election rally in Narkatiyaganj in West Champaran district to canvass for the Congress candidate Md Fakhruddin.
It was second joint campaigning by Nitish Kumar, Lalu Prasad and Congress leaders for the Bihar bypolls since these parties struck an electoral alliance early this month.