"RJD will support JD-U's two candidates in the Rajya Sabha by-election to defeat BJP's game plan," he said at a press conference after holding a meeting of the RJD legislature party.
The former chief minister said his party decided to support JD-U to forestall any chance to communal forces to play their dirty politics.
Pawan Varma and Gulam Rasool Balyawi will face two independents- Anil Sharma and Sabir Ali in Thursday's Rajya Sabha poll.
On Saturday, Nitish had approached Lalu for support to ensure victory of party candidates in the bypoll for two Rajya Sabha seats in Bihar on June 19.
While accusing BJP as the main conspirator in the RS poll, Nitish had said, "I have talked to Lalu Prasad, Congress state chief Ashok Choudhary, its legislature party leader Sadanand Singh and CPI secretary Rajendra Singh seeking their support for victory of two JD(U) candidates in the bypoll to the Rajya Sabha in Bihar."
Two political rivals come together
The prevailing political circumstances has led two staunch rivals- Nitish and Lalu- to fight a common enemy in BJP.
It was probably the first time in two decades that Nitish Kumar has publicly admitted that he sought support from the RJD chief after he and Lalu Prasad parted ways in 1994.
In the past, both Nitish and Lalu have openly criticised each other during rallies.
Lalu's decision to support JD-U is seen as the first step for a likely realignment of RJD-JD-U to take on a surging BJP in Bihar, despite reservations expressed by some RJD legislators and senior leaders like Raghuvansh Prasad Singh.
In February, when 13 out of 22 RJD MLAs had defected to JD(U), then Lalu had alleged that Nitish hatched a conspiracy to break his party.
During election campaigns, Lalu had even said, "Nitish Kumar bhaag gaya, Lalu Prasad jaag gaya."
BJP to discuss strategy
Meanwhile, the Bihar BJP legislature party will meet at the former deputy chief minister Sushil Kumar Modi's residence later this evening to chalk out its strategy for the Rajya Sabha bypoll on Thursday.
The Bihar BJP's 'wait and watch' stand is likely to keep both the JD(U) leadership and the rebel group with about 20 MLAs on tenterhooks till the actual voting takes place tomorrow in the state legislature premises.
The BJP has 84 MLAs, while the ruling JD(U) has 117 MLAs, but about 20 of them have turned rebels under the leadership of Barh MLA Gyanendra Singh Gyanu.