This paper aims to outline, first time based on research, the relationship between the two major thinkers of Poalei Zion (P.Z.): Ber Borochov and Nachman Sirkin. In his paper `Our Platform` (1906) Borochov formulated the theory of stages regarding settling Erez Israel: at the beginning the settlement is to be based on capitalist economy. Only later will the proletariat struggle against bourgeoisie and establish socialism. Years before, in 1898, Sirkin developed a unique synthesis of socialist Zionism, based upon non-Marxist foundations. It was upon this basis that Sirkin conceptualized his utopic realism: Socialism was to be realized through colaboration between the working class and the bourgeoisie. The working class should lead the nation to social justice based on cooperative socialism. Syrkin`s influence on the American P.Z grew increasingly. During t World War I Borochov stayed in New York, and his positions transformed. His biographer, Matityahu Mintz, failed to provide a convincing explanation of this shift. In this paper I will demonstrate the growing impact of Sirkin on Borochov, which was exemplified in Borochov`s "new tunes" on his return to Russia in 1917. This shift included dissociation from his `theory of stages` and a coming closer to ideas of constructive socialism and cooperation. P.Z leadership in Russia rejected this change. They remained `Borochovists` and silenced the real Borochov, who died shortly after. Sirkin, however, continued to develop Borochov`s way. In the P.Z. convention in Stockholm, 1919, he outlined a large-scale concept of constructive socialism. Thus he showed P.Z. a realistic and relevant vision. This vision had a profound influence upon Eretz Israeli socialism.