Congress chief spokesperson Randeep Surjewala said that Mr Kumar had pledged never to join hands with the BJP and alleged that his mask of "big talks and self-professed honesty" stands exposed, with his sole principle being "power at any cost, power at all costs".
Recalling that Prime Minister Narendra Modi had questioned Mr Kumar's "political DNA" ahead of 2015 elections, he said, "It has been proved today that the political DNA of BJP and JD(U) is identical - opportunism, treachery, hunger for power and disregard of people's mandate".
"In Bihar, every political and democratic principle was murdered in broad daylight and a government of BJP-JD(U) has been formed today, which is neither people's mandate nor does it have the authority to rule. This is a black day indeed for Indian democracy," the Congress leader said.
Mr Surjewala said it is an affront to the people's mandate and a "double whammy" for democracy that within 20 months, the politics of opportunism was repeated "when a party rejected by the people and a leader hungry for power at any cost formed a government overnight".
He said in November 2015, the people of Bihar rejected the leadership of Narendra Modi and the policies of BJP and the mandate of people was in favour of the policies, principles and leadership of 'Mahagathbandhan'.
"Nitish Kumar and JD(U) were never given the mandate to rule Bihar alone by the people and the BJP was decisively rejected. The new BJP-JD(U) government is founded on political conspiracy rather than people's mandate and has no right to continue even for a day as it is an affront to the mandate given by people of Bihar," he said.
Asked about Rahul Gandhi's claims that he knew of the plan for 3-4 months, Mr Surjewala said the Congress leadership believed in the words of Nitish Kumar who proclaimed never to join hands with the BJP.
He also accused Nitish Kumar of changing not only his leanings but also his "dharma". He said this is not the first time he has done so and cited previous cases when he ditched many a leader.
On opposition unity, he said, it is intact and all political parties are together.