He dared the Yadav loyalists, who had claimed that the "original" JD(U) was with them, to "break" the party.
"Break the JD(U) legislative party if you have the might...Prove a two-thirds majority within the party for breaking it, otherwise your membership will go," Kumar said, throwing a challenge to his detractors, led by Yadav.
"Why did you (Yadav) not speak up in 2013 when the party broke away from the NDA? You were the president of the party at that time," he said.
Addressing an open session after the JD(U) national council and national executive meets, Kumar said speaking up after the development was of no importance.
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Making a veiled attack on Yadav, who held a parallel 'Jan Adalat' meeting in Patna today, instead of attending the JD(U) national executive meet at Kumar's official residence, the chief minister said, "Only one thing is being run jointly nowadays -- promoting family interests in politics."
Media reports here claimed that one of the reasons why Yadav had moved closer to RJD supremo Lalu Prasad was to field his son from the Madhepura Lok Sabha constituency in 2019 with the RJD's help.
Kumar recounted several episodes to highlight the "political respect" shown by him to Yadav.
"In 2004, when he (Yadav) lost the Lok Sabha election from Madhepura, I sat with the then party chief George Fernandes for more than two hours to persuade him to send Yadav to the Rajya Sabha," he said.
He also hit out at Lalu and the RJD for levelling various allegations against him after the break-up.
Kumar mentioned that JD(U) principal general secretary Tyagi had pointed out that 16 of the 20 state unit chiefs of the party, all the 71 party MLAs and 30 MLCs of Bihar and the office-bearers of various committees were siding with him.
"Why did you not speak up when you were sent to the Rajya Sabha in 2012 with the help of the BJP?" he asked Ansari.
Kumar condemned Yadav's supporters for "creating a nuisance" outside his 1, Anne Marg residence earlier in the day. "Do you wish to lead Bihar on the strength of such hoodlums?" he asked the former JD(U) president.
Kumar justified the decision to part ways with the RJD and refuted the argument that it was an "insult" to the people's mandate given to the Grand Alliance in 2015.
"The mandate was to work for the people of Bihar and not to carry the load of sins of such people," he added.
Deputy Chief Minister Sushil Kumar Modi has levelled a series of corruption allegations against Lalu and his family.
On the RJD's statement questioning the support base of the JD(U), Kumar said, "It was due to the votes of the JD(U) that you became the largest party (in Bihar). But as the RJD's votes were not transferred in sufficient numbers, some of the JD(U) candidates lost (in the 2015 Bihar Assembly polls)."
"And those 22 seats were won when Ram Vilas Paswan was with you. Now, he is with us. Think of your future performances," he cautioned the RJD.
On the JD(U)'s support base, Kumar said, "Whosoever the JD(U) goes with, they win," apparently referring to the NDA's victory earlier when the JD(U) was with the BJP and again in 2015, when the JD(U) had joined hands with the RJD and Congress and the Grand Alliance had won more than two-thirds of the seats in the 243-member Bihar Assembly.