The Reddy brothers have fielded their associate and former minister B. Sriramulu as an Independent candidate after making him resign both from the BJP and the assembly, to which he was elected in May 2008 from this constituency.
Bellary is about 300 km from Bangalore and the rural seat is reserved for the Scheduled Tribes.
Over 170,000 people, including about 86,000 women, are eligible to vote in the bypoll which has become a matter of political survival for the three Reddy brothers.
The brothers were a pampered lot in the BJP not long ago because of the financial clout they wielded from the billions of rupees they made from iron ore mining, most of it allegedly from illegal mining and export.
For the BJP, it is a prestige issue as it wants to show that it is the party and not the individual that matters.
A defeat has the potential to bring down the D.V. Sadananda Gowda ministry as the Reddy brothers claim the support of around 14 of the BJP's 119 lawmakers in the 225-member assembly that includes one nominated member.
The battle is essentially between Sriramulu and the BJP's nominee P. Gadilingappa, a Bellary businessman, who is contesting an assembly poll for the first time.
The Congress, hoping that it will benefit from the battle between the BJP and its once-favourite money bags, has nominated B. Ramprasad, who lost to Sriramulu in the 2008 elections.
The Janata Dal-Secular, the third major political party in the state, has not fielded a candidate and is indirectly supporting Sriramulu on the ground that its strategy is to defeat both the BJP and the Congress.
Counting of votes is Dec 4.
The most aggressive of the three brothers, Gali Janardhana Reddy, former tourism minister, is directing the battle against the BJP from his cell in Hyderabad's Chanchalaguda Jail.
He has been lodged there since Sep 5 in connection with illegal mining case in Andhra Pradesh.
The younger Reddy, Gali Somashekara, has been suspended from the BJP along with two Lok Sabha members and three state lawmakers for openly canvassing for Sriramulu.
The elder Reddy, Gali Karunakara, also a former minister, has been away from public gaze after he appeared before the Central Bureau of Investigation in Hyderabad for interrogation a few days after Janardhana's arrest.
Sriramulu has announced that he will float a new party after results are out Dec 4. He says he is waiting for Janardhana Reddy to come out of jail to finalise details of the new outfit.