The initiators of the bloc were Mordechai Anielewicz, Józef Lewartowski (Aron Finkelstein) from the Polish Workers' Party, Josef Kaplan from Hashomer Hatzair, Szachno Sagan from Poale Zion-Left, Jozef Sak as a representative of socialist-zionists and Izaak Cukierman with his wife Cywia Lubetkin from Dror. The Jewish Bund did not join the bloc though they were represented at its first conference by Abraham Blum and Maurycy Orzech. According to Hersz Berlinski and Izaak Cukierman, the Bund did not join because they were waiting for a general socialist organization to be formed which would encompass non-Jewish Poles.