The 'Mahagathbandhan' (Grand Alliance) currently comprises the RJD and the Congress.
The announcement was made by Manjhi outside his residence here, in the presence of RJD leaders Tejashwi Yadav, Bhola Yadav, Tej Pratap Yadav and others who had called on the former Bihar chief minister.
"We will quit the NDA and join the Mahagathbandhan. A formal announcement to this effect will be made later tonight when the reasons for this major decision will also be made public," Manjhi told reporters.
The development came two days after Manjhi had demanded that the NDA field at least one member from his party for the six Rajya Sabha polls in the state scheduled next month and warned that the HAM would not campaign in support of candidates of the BJP and the JD(U) for by-polls in two assembly and one Lok Sabha seat scheduled for March 11.
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Kumar, who was then with the Grand Alliance, returned to the NDA in July last year.
Asked whether Manjhi has been promised a Rajya Sabha seat, Tejashwi Yadav said "decisions regarding Rajya Sabha candidates will be taken at an appropriate time.
"Today's development underscores the simmering discontent within the arrogant NDA. Upendra Kushwaha (Union minister and RLSP chief) has expressed his discomfiture in the BJP-led alliance many times. It is time for people like him to take a decision," Tejashwi said.