Former JD(U) leader Ajay Alok, a regular on TV debates, joined the BJP on Friday in the presence of Union minister Ashwini Vaishnaw.
Joining the BJP is like coming to a family, Alok said, vowing to contribute to the "Modi mission".
The country is today moving in the same direction as the prime minister, he told reporters.
He has been a strong defender of the BJP's policies and is seen by the party as a sharp and articulate voice.
He was expelled by the JD(U), the ruling party in Bihar, last year as it believed him to be close to R C P Singh, the party's former president who quit following a fallout with its supreme leader and Chief Minister Nitish Kumar.
Vaishnaw lauded Alok's contribution and praised him as a sharp analyst.
Alok took a swipe at Kumar, saying he is an architect who is destroying his own work by compromising on law and order, corruption and communalism.
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