"Parties which does not exist in Parliament but have strength outside Parliament can be members of the bloc," Kumar told reporters on his arrival from Delhi.
The JD(U) leader met top leaders like former Prime Minister H D Deve Gowda, CPI(M) general secretary Prakash Karat, CPI leader A B Bardhan and Forward Block general secretary Debabrata Biswas in Delhi to take the matter forward.
About the grouping's future plans for the forthcoming Lok Sabha elections, Kumar said the parties were yet to decide on the Prime Ministerial candidate and at the moment the front was working on a "comprehensive and solid agreement" over federalism and secularism as the basis of its constitution.
He exuded confidence that the proposed front would be the biggest coalition as people have lost their hopes on both Congress and the BJP.
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The Left Front was the prime initiator of the efforts for the formation of an alternative front and JD(U) was actively supporting the move, he stressed.
Nitish took an apparent dig at Narendra Modi for remarks of a Gujarat minister linking migration from Bihar and Uttar Pradesh to surge in the number of poor in the state over the last decade.
"For this they are blaming people from Bihar and Uttar Pradesh going there... Like Maharashtra, voices against people of Bihar have started in Gujarat also," he criticised.