The Bharatiya Janata Party had won two of the six constituencies in 2009 in alliance with the Janata Dal-United (JD-U) which too won from two places.
The May 12 balloting will be crucial for the BJP to counter a resurgent Rashtriya Janata Dal (RJD) of Lalu Prasad and to push Chief Minister Nitish Kumar's JD-U to the third spot.
The BJP and the JD-U, which ended its 17-year alliance with the BJP last year, are contesting the election without each other's support for the first time since 1996.
Of the six constituencies, the RJD won only in Vaishali, thanks to the appeal of its candidate Raghuvansh Prasad Singh.
A total of 90 candidates, including five women, are in the electoral fray Monday. Polling will be held in 8,582 polling stations. More than nine million electorate is eligible to vote.
Addressing meetings in all six constituencies, BJP's prime ministerial candidate Narendra Modi sought to emphasize that he came from an extremely backward caste.
He also repeatedly targeted Congress leaders Sonia Gandhi and Rahul Gandhi and Congress campaigner Priyanka Gandhi for their remarks blaming him for "neech politics" remarks.
Modi's caste card is widely as an attempt to woo Bihar's backward castes, Dalit and Mahadalits.
The BJP is confident of support from the upper castes - Brahmins, Bhumihar and Rajputs - and also hopes to garner support of backward castes and Dalits, thanks to its alliance with the Lok Janshakti Party (LJP) of Ram Vilas Paswan and the Rashtriya Lok Samata Party (RLSP) of Upendra Kushwaha.
Modi's caste is used as an attempt to woo Bihar's backward castes
But with the Congress joining hands with the RJD and with Lalu Prasad claiming that his traditional Yadav-Muslim support base was in place, this election will be a major challenge to the BJP.
JD-U leaders are confident of doing better than last time in the final round of polling.
The party is banking on Nitish Kumar's development card and his "social engineering" of extreme backward castes, Mahadalits and Muslims, spokesman Neeraj Kumar said.
Prominent leaders in the fray Monday are former union minister Raghuvansh Prasad Singh, Raghunath Jha, filmmaker Prakash Jha, Rama Singh, Satish Dubey Hina Sahab, wife of tainted and jailed former MP Mohammad Shahabuddin, Annu Shukhla, wife of jailed former legislator Munna Shukhla.
In West Champaran, Prakash Jha is trying his luck for the third time. Pitted against him is outgoing MP Sanjay Jaiswal of the BJP and Raghunath Jha of the RJD.
In Siwan, Hina Sahab (RJD) is contesting against BJP's 2009 winner Om Prakash Yadav.
In Vaishali, RJD's Raghuvansh Prasad Singh has been taken on by BJP ally and LJP candidate Rama Singh. Raghuvansh has won the seat five times in the past.