Reacting to Kumar's comment yesterday that efforts were on for a long time to end his political career, Modi said it would "definitely" happen if he continued to stay with RJD and Congress.
"If you (Kumar) stay with Lalu Prasad and Congress you will definitely be killed politically," he told reporters.
"RJD is known for its corrupt practices while the Congress rule was marred with scams," said Modi, the Parliamentary board chief of Bihar unit of BJP.
He alleged that due to the alliance with RJD and Congress, Kumar's effective step against corruption through Special Court Act which has provision for confiscation of property of a guilty has "slowed down".
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Asked if there is any chance of reunion of JD(U) and BJP, the Leader of Opposition in Bihar Legislative Council was quick to say: "If a proposal comes from JD(U), the party would consider."
"Politics is a game of possibilities ... You never know when one comes with whom in politics," he added.
He, however, feigned ignorance about media reports of Kumar meeting BJP President Amit Shah and talking to Prime Minister Narendra Modi about his support for demonetisation.
"These are high level issues to which I have no access," he said.
"If (Sushil) Modi wants he can take Kumar home on his lap," she told reporters emerging from the Legislative Council after it adjourned for the day.
Kumar had led JD(U) to severe ties with BJP in June 2013 after 17 years over elevation of Narendra Modi as PM candidate. He joined hands with RJD and Congress before the Bihar polls last year and romped back to power.