For the RJD, it is a prestige fight as Gopalganj is the home district of party chief Lalu Prasad.
Bihar's ruling coalition held on to one seat but lost in another seat by a hair in a prestige battle with the BJP just months after Chief Minister Nitish Kumar dumped the BJP and formed a new government with Tejashwi Yadav's RJD.
Bihar Deputy Chief Minister Tejashwi Yadav's RJD retained the Mokama assembly seat but failed to wrest the Gopalganj seat from the BJP. The RJD lost by just 2,000 votes.
Gopalganj voted after the death of BJP MLA Subhash Singh, whose wife Kusum Devi contested as the party's candidate. She defeated the RJD's Mohan Gupta.
The RJD campaigned aggressively for what became a prestige battle in Gopalganj, the home district of party leader and former Chief Minister Lalu Yadav, the father of Tejashwi Yadav.
But the ruling coalition's votes were dented by candidates of Asaduddin Owaisi's AIMIM and Mayawati's Bahujan Samaj Party (BSP). The AIMIM won around 12,000 votes and the BSP took away 9,000, helping the BJP scrape through.
The RJD defeated the BJP in Mokama by over 16,000 votes.
RJD candidate Neelam Devi contested the Mokama by-polls after the seat fell vacant following her husband Anant Singh's disqualification over corruption charges.
Mokama has been the stronghold of Anant Singh since 2005. He won the seat twice as a candidate of Nitish Kumar's Janata Dal United. The BJP contested in Mokama for the first time; it had always left the seat to allies.
The Nitish Kumar-led alliance had hoped to win both seats as a sign of public support for the Chief Minister's decision to snap ties with the BJP and form a new government with the RJD, Congress and other opposition parties.
Nitish Kumar is prominent in the line-up of possible opposition faces to take on the BJP and Prime Minister Narendra Modi in the 2024 national election.