The majority stayed within the mainstream party but a minority who supported the Bolshevik path became known as Left Socialist-Revolutionaries. Maria Spiridonova was a prominent leader of this group. They, in effect, split from the main party. The split was not completed before the Russian Constituent Assembly elections; the first meaningful electoral test between the parties in the peasant soviets a few weeks after the Assembly elections showed the parties had roughly equal support among the peasantry.
The Left Socialists left the Socialist Revolutionary Party due to their support for the overthrow of the Alexander Kerensky government. The Left Socialists would later support the Bolsheviks who came to overthrow the provisional government.
The Left SR party became the coalition partner of the Bolsheviks in the Soviet Government (the Council of People's Commissars) after the October Revolution of 1917, heading the People's Commissariats for agriculture, justice, posts and telegraph, etc. They later resigned their positions in protest at the signing (3 March 1918) and ratification of the Treaty of Brest-Litovsk, but continued for a time to co-operate with the Bolsheviks at other levels.
The Left Socialist Revolutionaries had participated in the Bolshevik government until July 1918 at the fifth Congress of the Soviets, where they were expelled. The Left Socialists had criticized the Bolsheviks for their repressions on trade union freedoms, ending factory worker councils, and support of the Treaty of Brest-Litovsk.
In mid-July 1918, the LSR tried to incite an uprising in areas ceded to Germany by the Brest-Litovsk Treaty, so their supporters were involved in the assassination of German ambassador Graf von Mirbach in Moscow on July 6, and Field Marshal von Eichhorn, commander of Army Group Kiev and military governor of Ukraine, on July 30.