"Delhi will now see the leadership of mahagathbandhan. All coalition partners should start preparations," Mr Yadav tweeted after Bihar Congress chief and state education minister Ashok Chaudhary wished him.
The RJD chief also reminded Trinamool Congress chief Mamata Banerjee that they stand in solidarity for the "great cause ahead" after she extended greetings to him.
As Lalu Yadav turned 70 day, Bihar Chief Minister Nistish Kumar drove to 10 Circular Road to wish him and praised his "immense contribution to Bihar politics and society".
Ever since a Nitish-led 'mahagathbandhan' of RJD, JD(U) and Congress handed down a stunning defeat to BJP in 2015 Bihar Assembly election, a grand alliance is being seen as the only way to stop the saffron juggernaut.
While similar attempts to forge a coalition in Uttar Pradesh fizzled out, the upcoming Presidential election has seen fresh moves for opposition unity against the BJP.
A stream of well-wishers comprising legislators and party workers extended greetings to Mr Yadav.
Later in Lalu Yadav's presence, Nitish Kumar dedicated two road bridges over the river Ganga to the people of the state.