"If you are happy with the development work of my government, vote us as a reward," Kumar told an election rally here in favour of JD(U) candidate P K Shahi.
The by-poll is scheduled on June 2 and will see a triangular contest between JD(U), RJD and Congress.
Highlighting achievements of the NDA government, he said "My government bettered the environment in the state to pave way for development."
"Earlier since the environment was bad, there was no development work in the state. But now since the environment has bettered and progressive works have been carried out on a large scale," he said in a veiled attack on rival RJD rule in the state.
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"People are talking how a farmer of Bihar broke world record of China in paddy cultivation," he said, adding the people in backward areas are now drawing inspiration from the development in the state.
"However, some people are feeling bad about the positive changes in the state," he said in an indirect attack on political rival Lalu Prasad and urged the voters not to let the momentum of growth be disrupted.