Singh, who served as minister all through the JD(U) government except for the current spell of Nitish Kumar starting from February this year, took membership of RLSP from its Chief and Union minister of State Upendra Kushwaha.
Singh hails from Saran district which is considered stronghold of RJD President Lalu Prasad.
Welcoming Singh in the RLSP, Kushwaha said many leaders having their roots in Samata Party are feeling "suffocated" under the leadership of Chief Minister Nitish Kumar who "changed his track for the sake of power."
"I was looking for an opportune time to say goodbye to Nitish Kumar for bringing back jungle raj against which we had fought and the time has come now," he said.
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Kushwaha said formal talks on seat-sharing in NDA has not begun.
He, however, exuded confidence it would be sorted out amicably soon.
Asked how many seats RLSP was expecting, Kushwaha, who is Union Minister of State for HRD, refused to spell the number saying "it will be done in the meeting of NDA."
Among allies it had mooted 74 seats for LJP and 67 for itself. Jitan Ram Manjhi's Hindustani Awam Morcha (HAM) was not part of NDA at the time of the national executive.
"Now with entry of Manjhi's party, some alteration is natural. But, still we expect allies getting good representation in NDA," he said.
On NDA not declaring a Chief Minister candidate, Kushwaha whose name was floated by his party for CM post earlier, said "its part of NDA strategy."
He slammed Kumar and Lalu Prasad for criticising PM on special package of Rs 1.65 lakh crore which would bring a major turnaround in the state.