Here are the latest developments:
"We have already decided to work together. Now the next step is a merger to form one party. Mulayam Singh will work out modalities in that direction," Mr Kumar said. His Janata Dal (United) and partner Lalu Prasad Yadav's Rashtriya Janata Dal are among six members of the new front.
Mr Kumar emphatically denied that the move stemmed from a "fear of Narendra Modi" and the ruling BJP, which has been making big inroads in the states that these parties dominate. But he made it clear that attacking the Modi government tops the group's agenda.
"It was being said that everyone will have 15-20 lakh in their accounts so what happened to that?... Promises were made during elections of providing employment... These issues will be topmost of our protest agenda," Mr Kumar said.
He said the Samajwadi Janata Dal will hold a 'dharna' or sit-in in Delhi on December 22 against what he called the government's failure to bring back black money stashed abroad and on the issues of farmer suicides and rising unemployment.
Mr Kumar also said people from outside the "Janata Parivaar who may have the same feelings" could also join the front later.
His announcement came after a meeting at Mulayam Singh Yadav's Delhi residence. At the meeting were, Nitish Kumar, his party colleague and JD(U) President Sharad Yadav, Lalu Prasad Yadav and JD(S) chief HD Deve Gowda.
At the first such meeting last month, these leaders had met over lunch and decided to work as a pressure or influence group in the Winter session of Parliament.
Today Mr Kumar said, "It has been decided that we should have unity and we have to take it further both inside as well as outside Parliament."
Most of these regional parties fared poorly in the national elections, as the BJP and its partners seized a sizeable majority in the Lok Sabha.
Mulayam Singh Yadav, whose party rules the crucial Uttar Pradesh, has won just five seats in the Lok Sabha. Nitish Kumar's JD(U), the ruling party in Bihar, has only two.
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