Often seen as a possible opposition candidate for prime ministership in 2019 Lok Sabha polls, he said no serious attempt is currently being made to bring together non-BJP parties.
A grand alliance of the opposition parties at the national level is certain to win the elections but no such scene was in sight, he said.
He asserted there is no problem in the ruling grand alliance in Bihar and mocked speculation about his growing proximity with BJP, saying he would not offer denial on all kinds of reports as denial is 50 per cent confirmation.
"This 'notebandi' is a positive step which will lead to benefits. It will happen," the JD(U) President said, terming corruption and black money as a "big sin and festering wound" which must be eradicated.
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"Severe steps will have to be taken. That is why we offered our support to this (demonetization). It is a positive beginning... Whenever there is an attack on corruption, I will support it," he said.
Asked about Yadav and other opposition parties raising the issue of people's difficulties, he said, "I don't speak about it because those facing difficulties should be speaking. No such voice is being heard. People are in difficulty but the poor thinks he is facing problem today but a lot of money of the rich is sinking."
Kumar, however, added that demonetization alone will not
end black money and asked Prime Minister Narendra Modi to also launch a crackdown on benami properties immediately and target tainted money hoarded in assets like gold and diamonds. Now is the right time to do so, he said.
Kumar's remarks in support of demonetization, which has united most of the opposition parties against it, came after Yadav hit out at Modi dispensation over the issue.
Yadav alleged the decision is aimed at recapitalising banks that were "sinking" due to Rs 8.5 lakh crore of Non- Performing Assets and its goal is not to target black money.
Kumar, however, did not touch upon the implementation part of demonetization, merely saying that his government does everything with due preparation and so should the Centre.
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Taking a dig at West Bengal Chief Minister Mamata
Banerjee, one of the most vocal critics of demonetization who had attacked him at a public meeting in Patna, Nitish Kumar said too much aggression affects perception which is important in democracy. "People will ask why somebody is so aggressive."
"I support something which I agree with and oppose which I do not... Media does not look at things in a simple and normal way and does things for publicity," he said, referring to political speculation over his support to demonetisation.
"Just by having ambition one does not become Prime Minister... My national ambition is to make JD(U) a national party like the original Janata Dal which my party represents now."
He also batted for nation-wide prohibition, which he has enforced in Bihar, saying a lot of black money is generated by the liquor trade and now is the right time to go after it.
Condition in many parts of Delhi is worse than villages in Bihar, he said.
His party had supported AAP in the last assembly polls.
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In an indication that JD(U)'s relationship with Arvind Kejriwal's AAP is no longer as it used to be, Nitish Kumar asked party leaders to start preparing for municipal and subsequent state elections.
"Biharis living in Delhi are not seeking somebody's mercy or begging... They are not burden on Delhi but have carried its burden. You should unite," he said.
Many leaders told him that JD(U) should have carried on with the work it had started earlier as they joined other parties. It had not contested the last assembly polls and supported AAP.
He referred to poor living conditions in several parts of Delhi and said the city will come to a halt if Biharis do not work for a day.
Top JD(U) leaders from Bihar and its MPs will take part in city unit's programmes to build the party.
Kejriwal, like Mamata Banerjee, has taken a strident stand against demonetisation.