The Socialist Revolutionary Party (SR; Russian: Па́ртия социали́стов-революционе́ров, romanized: Pártiya sotsialístov-revolyutsionérov,, lit. 'Party of Socialists-Revolutionaries') was a major socialist political party in the late Russian Empire, during both phases of the Russian Revolution, and in early Soviet Russia. The party members were known as Esers (эсеры, esery, from "SRs").
The SRs were agrarian socialists and supporters of a democratic socialist Russian republic. The ideological heirs of the Narodniks, the SRs won a mass following among the Russian peasantry by endorsing the overthrow of the Tsar and the redistribution of land to the peasants. The SRs boycotted the elections to the First Duma following the Revolution of 1905 alongside the Russian Social Democratic Labour Party, but chose to run in the elections to the Second Duma and received the majority of the few seats allotted to the peasantry. Following the 1907 coup, the SRs boycotted all subsequent Dumas until the fall of the Tsar in the February Revolution of March 1917. Controversially, the party leadership endorsed the Russian Provisional Government and participated in multiple coalitions with liberal and social-democratic parties, while a radical faction within the SRs rejected the Provisional Government's authority in favor of the Congress of Soviets and began to drift towards the Bolsheviks. These divisions would ultimately result in the party splitting over the course of the fall of 1917, with the emergence of a separate Party of Left Socialist Revolutionaries. Meanwhile, Alexander Kerensky, one of the leaders of the February Revolution and the second and last head of the Provisional Government (July–November 1917) was a nominal member of the SR party but in practice acted independently of its decisions.
