Manjhi announced his decision in presence of RJD leader Tejashwi Yadav.
Within hours, former Bihar Congress chief Ashok Choudhary, along with three other party MLCs, announced they they would quit the party and accused its general secretary C P Joshi of having treated them as "objects" that were supposed to be "used and thrown".
"Joshi was kind of pushing me out of the Congress," Choudhary alleged.
Reacting to Qadri's claim, Choudhary said, "A letter expressing our desire to join the JD(U) was handed over to the officiating chairman of the Legislative Council, Haroon Rashid, hours before the so-called letter of expulsion was issued by Qadri."
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He also claimed that a number of party MLAs were in touch with him and were likely to follow his suit.
He alleged that, earlier, several Congress leaders from the state such as Anil Sharma, Mehboob Ali Qaisar and Ram Jatan Sinha had been meted out shabby treatment.
Choudhary said when Nitish Kumar was heading the Grand Alliance government, he was asked by the Congress to have a close relationship with Kumar.
Meanwhile, Manjhi, while addressing a press conference in the evening, spoke elaborately on the reasons behind his decision to quit the NDA.
He alleged that the "faulty" sand mining policy of the Nitish Kumar government had stalled construction activities thus adversely affecting workers from the Mahadalit section.
"Mahadalits are also getting a rough deal under the prohibition law. But, whenever I tried to take up the matter with the chief minister, he made fun of me," Manjhi alleged.
Manji also claimed that the BJP had made no effort to save his government when he had fallen out with Nitish Kumard.
"Rather, the BJP set up rebel candidates against my party nominees, including my son, in the last Assembly polls, resulting in their defeat," the former chief minister said.
Reacting to Manji's decision, JD(U) Spokesman Sanjay Singh said, "His exit will have no impact at all as he already stood defamed because of his political inconsistency."
Manjhi had earlier served an ultimatum to the NDA and had said that he would not campaign for the alliance in the upcoming bypolls to one Lok Sabha and two Assembly seats, if he was not given at least one of the six Rajya Sabha seats in the biennial elections scheduled to be held next month.