BJP President Amit Shah, who is in Bihar for the first time after assuming charge of the party, attacked Kumar, a former Bihar chief minister, as well RJD Chief Lalu Prasad, with whom he has joined hands.
Shah chose the birth anniversary of veteran socialist leader and former Bihar Chief Minister Karpoori Thakur to attack his two principal disciples -- Prasad and Kumar -- whose parties, both offshoots of the old Janata Parivar, have come together.
He accused Prasad and Kumar of going against the teachings of Thakur by joining hands with Congress, the party which, Shah said, the veteran socialist leader had fought all his life.
"To gain power, they are taking the help of Congress...You (Kumar, Prasad) have no right to take the name of the late Karpoori Thakur," Shah said at the meeting held at the Veterinary College grounds.
Bihar would go to the hustings later this year. The state would also be a major test for regional parties as they try to regroup against BJP, which won a thumping victory in the Lok Sabha election.
"BJP's Vijay Rath will roll in the state later this year," said Shah, who was flanked by Union ministers hailing from Bihar and senior state leaders like Sushil Kumar Modi and Nand Kishore Yadav.
As Shah was in the Bihar state capital looking to push BJP's cause in Bihar, Kumar was away to Azizpur village in Muzaffarpur district to meet victims of Sunday's arson.
Shah said BJP had always respected Kumar's wishes and made him the chief minister to keep Prasad and Congress away from power in Bihar.
"But Nitish Kumar stabbed the peoples' mandate in the back and joined hands with Prasad and Congress to gain power," he said.
BJP was an equal partner in bringing Bihar back on the track under NDA rule in the state, he said.
"There was 'sushasan' (good governance) in Bihar because Nitish Kumar and Sushil Kumar Modi were together," he said.
Reacting to Prasad's criticism of the Narendra Modi government for allegedly failing to stop frequent firing and infiltration at the borders from the Pakistani said, Shah said
"Lalu-ji your eyesight has become weak."
There is a 'marked difference' in the situation at the borders under the previous UPA and present NDA times, Shah said.
"Earlier, Pakistan used to start and end firing on the borders but, now, though the firing is started by Pakistan, its end comes from India's firm response," said the BJP chief.
Criticising the coming together of Kumar and Prasad, he said Bihar was witnessing a return of 'jungle raj', a situation which was faced in the state during RJD rule.
Shah said if BJP is voted to power it will make Bihar the 'No.1' state in the country in five years and provide round-the-clock electricity by signing MOUs with power companies.
"If Bihar and West Bengal get good governments which share the vision of Prime Minister Modi, both states would run fast on the path of development," he said.
Claiming that the amount of work done by the NDA government to ensure the return of black money was unparalleled, he said that at its first Cabinet meeting itself it had constituted an SIT in this regard.
"Under Congress, SIT used to be set up to protect persons found involved in wrong-doing," he alleged.
Meanwhile, Sushil Modi said BJP and Jan Sangh had played a significant role in making Karpoori Thakur the chief minister of Bihar.
Union ministers, including Ravishankar Prasad, Radha Mohan Singh, Ramkripal Yadav, Rajiv Pratap Rudy and Giriraj Singh, welcomed Shah.