In the race are Prince Raj, the 26-year-old candidate of Paswan's Lok Janshakti Party (LJP), a BJP ally, and former MP Maheshwar Hazari, 44, of the ruling Janata Dal-United (JD-U).
Kalyanpur is an assembly segment of the Samastipur Lok Sabha constituency, represented by Prince's father Ram Chandra Paswan, a brother of Ram Vilas Paswan.
Hazari, who once represented Kalyanpur in the Bihar assembly, lost to Ram Chandra Paswan in the 2014 general elections by 73,000 votes. Earlier in 2009, Hazari had defeated Paswan.
Hazari is the son of prominent socialist leader, the late Ram Sewak Hajari, a six-time legislator from Kalyanpur. Hazari senior is a cousin of Ram Vilas Paswan, was a former MP, and died in 2012 at age 76.
Kalyanpur has 2.83 lakh voters including 1.31 lakh women. It is dominated by Bhumihar, Kushwaha and Paswan communities.
In the last assembly polls, the JD-U's Razia Khatun defeated RJD's Manoj Kumar Yadav by 15,402 votes. That was when the JD-U was allied to the Bharatiya Janata Party. Now the JD-U and the RJD are in the same boat, bitterly ranged against the BJP and its allies.
"I voted for Modiji in the 2014 parliamentary elections as there was a Modi wave," businessman Mohammad Tabrrej told IANS, referring to Prime Minister Narendra Modi.
"But now the situation is different. The contest is between Hazari and Prince," he said. "Is baar to 'hawa' Nitishji ki hai" (Now the conditions favour Nitish).
The one man who doesn't agree is Jiwach Paswan, a candidate of the Communist Party of India-Marxist Leninist. Jiwach Paswan insists Kalyanpur is witnessing a triangular contest.
"Both the JD-U and LJP candidates belong to political families. People of Kalyanpur will reject dynastic politics. Only the CPI-ML can do this by defeating the 'chacha' and 'bhatija'," he said.
Considering the political significance of Kalyanpur, a reserved constituency, both Chief Minister Nitish Kumar and RJD chief Lalu Yadav started their election campaign from here.
Prince, who holds a Masters degree in international business from London, is banking on Prime Minister Modi's charishma other than the caste equations. He seems to have the support of the young voters.
"We have formed a Navyuvak Sangh and have decided to support Prince," said Sanjay Rai, who sells milk in the area.
"Laluji has done nothing for the youth. In the name of youth he promoted his sons. He recently remarked that Hindus eat beef. I am a Yadav, and Yaduvanshis can never eat beef," he added.
The Nationalist Congress Party, the Bahujan Samaj Party and the Jharkhand Mukti Morcha have also fielded their candidates in Kalyanpur.