"How can I think of contesting the Lok Sabha elections from his (Kumar's) side?" Tiwari said in a letter written to Bihar JD(U) President Bashistha Narayan Singh.
"I know very well that Nitish is asking me to contest the Lok Sabha elections not for my victory, but to get me defeated," he said.
Tiwari said he had asked Kumar that he was willing to fight LS poll when JD(U) had separated from BJP in June last year."But he did not say anything."
The JD(U) leader, who had praised Narendra Modi during party convention at Rajgir on October 28-29 in the chief minister's presence, claimed that since then Kumar was looking to settle scores with him.
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Tiwari had come to JD(U) from RJD and was made a RS member in 2008.
State party spokesman Neeraj Kumar told PTI that the senior leader's letter was contradictory.
Senior RJD leader Ramkripal Yadav sympathised with Tiwari and said he was always an asset to whichever party he joined.
Asked about Tiwari rejoining RJD, Yadav said that it would be decided by senior leaders.