RLSP President and MP Upendra Kushwaha said Kumar had once formed Samata Party to fight the "jungle raaj (lawless rule)" under Prasad in Bihar but was now back together with him.
"Now Kumar has come out in support of 'jungle raaj' and Prasad has extended his support to 'kushashan (misrule)'. He is exposed now," Kushwaha told reporters.
Kumar quit as the Bihar chief minister following his party's debacle in the just-concluded Lok Sabha polls and RJD extended its support to the new government which is to be led by Jitan Ram Manjhi.
Referring to the reported differences between Kumar and Sharad Yadav, Kushwaha claimed that the JD(U) president was in "touch" with senior NDA leaders and may soon cross over to the ruling alliance.
"He is upset with Kumar. It was clear even during the campaign. He has enjoyed good relations with BJP leaders in the past. He is in touch with alliance leaders," he said.
RLSP's Chief General Secretary Shivraj Singh said that the party would welcome it if Sharad Yadav came into the NDA fold as he was a senior leader of a national stature.
RLSP, which draws its support from a section of the backward castes, had consted three seats in Bihar and won all.