''We have decided to support Amit Sarkar,'' Leader of Opposition in the state Assembly Abdul Bari Siddiqui and state RJD president Ramchandra Purve told reporters.
Purve said RJD supremo Lalu Prasad had authorised the state unit of the party to take a decision on the subject.
Siddiqui said the decision had been taken to avoid division of secular votes and ensure defeat of communal forces.
''We tried to contact LJP supremo Ramvilas Paswan seeking his view on the matter, but he was not available. I am confident that the LJP will support the RJD's stand,'' Siddiqui said.
In-charge of Congress affairs in Bihar Gulchain Singh Charak had yesterday announced that his party had decided to fight the bypoll on its own.
While replying to a question as to whether his party would seek the support of RJD-LJP combine for the by-election, Charak had said, ''The secular parties should understand that the Congress is the lone alternative in the bypoll and should support its nominee.''
The RJD-LJP combine has not put up its candidate for the seat and rather said it would extend support to the candidate of a secular party.
The bypoll to Purnia Assembly constituency has been necessitated by the killing of ruling BJP MLA Raj Kishore Keshri allegedly by a woman at his residence in Purnia on January 4, 2011.
While BJP has fielded Keshri's widow Kiran Keshri, Congress has put up Ramcharitra Yadav as its candidate. Other candidates are CPI(M)'s Amit Sarkar, Islamuddin of CPI-ML and three independents.
Earlier, former state BJP president Radhamohan Singh, MP, had charged the RJD-LJP combine with entering into a tacit understanding with the Congress that forced the combine not to put up a candidate.
Black money and corruption, besides price rise would also be raised during the campaign for the seat considered prestigious for ruling NDA, Singh said.