In relation to the RSDRP, they argued for a looser, federal form of organisation, rather than a unitary centralised one. Liber was one of the Bund's representatives at the fateful Second Congress of the RSDRP in 1903, and the third most frequent speaker at the congress (after Lenin and Trotsky). He defended the Bund's demand to be recognised as an autonomous organisation within the RSDRP and as the sole legitimate representative of the Jewish proletariat in the Russian empire. This position was roundly rejected by both Lenin and I.O. Martov, soon to emerge as the leaders of the Bolshevik and Menshevik factions of the RSDRP. Martov had himself been a member of the Bund in the 1890s and one of Liber's former collaborators, but at the second congress, he supported Lenin in demanding the integration of the Jewish proletariat in an All-Russian Social-Democratic party. Martov proposed a more liberal criterion of party membership than Lenin, but by the time that issue led to the schism between Bolsheviks and Mensheviks, Liber and his fellow Bundist delegates had withdrawn in protest from the congress and from the RSDRP. It was the Bundists' exit that gave Lenin a slight majority at the congress (and hence enabled him to call his faction 'Bolshevik' - 'Majoritarian'). As the division between Bolsheviks and Mensheviks hardened, the Bund was increasingly drawn to the Menshevik side. The Mensheviks reversed their position on organisational federalism, making it possible for the Bund to rejoin the Menshevik wing of the RSDRP. In 1906, Liber represented the Bund at the 4th, Unification, Congress of the RSDLP in Stockholm when the Bund rejoined the party. At the 5th Congress in London in 1907 he was elected to the RSDLP Central Committee. During the abortive Revolution of 1905, Liber played a role as a Bundist representative in the soviets, even though he, like most SDs (including Lenin), initially greeted the new, spontaneous workers' organisation with some scepticism. When the Revolution petered out in 1907 and the autocracy reasserted its authority, Liber was one of those who advocated a more cautious, legalistic course of action for the RSDRP. Known as 'Liquidators' because they wanted to 'liquidate' the illegal underground organisation of the party, this group was fiercely opposed by Lenin as well as by 'Party Mensheviks' like Martov and Trotsky (then a Menshevik).