Here are the latest updates on this story:
Mr Modi took several digs at the Bihar Chief Minister without naming him. "Mr Paswan quit the NDA (BJP-led National Democratic Alliance) and expressed his anger. But in all these years, he always met us with affection. I know some leaders who meet you in private but break into a sweat at shaking hands in public," he sneered. (See pics)
In a speech that he began in the local Bhojpuri dialect, Mr Modi tore into the Nitish Kumar government for poor development. "In Bihar, it is news when there is electricity, not when there's a power cut," he said.
Mr Paswan returned the warmth, praising the man he has criticized for over a decade, and saying he should be Prime Minister. "Incidents happen, the riots happened in Gujarat 12 years ago, there have been nothing since then. But here in Bihar they happen every year," he said.
Mr Paswan walked out of the BJP-led NDA in April 2002, criticising Mr Modi's handling of the riots that tore through Gujarat earlier that year. The Lok Janshakti Party leader returned last week pledging support to Mr Modi's leadership. (Read: Modi is also from the extremely backward classes, says Paswan)
The BJP sees this as a neon sign that its prime ministerial candidate is no longer unacceptable to potential allies and a message for Nitish Kumar, who ended a 17-year alliance with the BJP last year over Mr Modi's elevation.
Nitish Kumar has accused Ram Vilas Paswan of opportunism. (Read)
BJP leaders CP Thakur, Giriraj Singh and Ashwani Chaubey, all known to be close to Mr Modi, are not attending the rally, reportedly as they are not happy with the tie-up. The leaders, however, cited "personal reasons."
Mr Modi last addressed a rally in Bihar in October. Multiple blasts ripped through capital city Patna minutes before he arrived at the venue of his hunkar rally, leaving seven people dead and over 80 injured.
The Bihar BJP, perpetually at loggerheads with the Nitish Kumar government, has accused it of trying to trip up their preparations by not allowing them to use both the residential and training grounds at the police lines for the Modi rally.
Mr Paswan quit a long-running alliance with Lalu Prasad's Rashtriya Janata Dal after they failed to agree on seat sharing. The RJD is close to tying up seat-sharing with the Congress for the state's 40 Lok Sabha seats. The BJP has agreed to let Mr Paswan's Lok Janshakti Party contest seven seats. Mr Paswan brings to the table about five per cent Dalit votes in Bihar. (Read: The importance of being Ram Vilas Paswan)
PM Modi Meets Party Staffers At BJP Headquarters BJP Accuses Congress Of Inciting Violence Against PM Modi With 'Maut' And 'Hinsa' Remarks Why BJP Lost Lok Sabha Polls In Uttar Pradesh - 6 Reasons In Party Report Windows Systems Restarting, Throwing Blue Screen Of Death Due To This Error Explained: What Is Causing The Dreaded 'Blue Screen Of Death' On Windows? Flights, Markets, Banks, Stock Exchange: Microsoft Outage Crippling Sectors Central Railways Jobs 2024: Applications For 2,424 Apprentice Posts Begin "Too Little Too Late": Ola CEO Slams Google For Slashing Maps' Fee Flights, Markets, Banks, Stock Exchange: Microsoft Outage Crippling Sectors Track Latest News Live on NDTV.com and get news updates from India and around the world.