A political organization created to fight the Bolsheviks and protect the Constituent Assembly. It was formed in Petrograd on November 23, 1917, it was initially regional. The Union included representatives of the following parties and organizations: Mensheviks, Socialist Revolutionaries (SRs), Popular Socialists, city and district Dumas of Petrograd, a number of trade unions, factories, and military units, some members of the disbanded Committee for the Salvation of the Motherland and the Revolution. Chaired by a Socialist Revolutionary Vasily Filippovsky.

The Union issued leaflets with appeals to various segments of the Petrograd population and soldiers of the Petrograd garrison. In December 1917, it held three working conferences attended by representatives of opposition parties, trade unions, military units, Obukhov and Putilov plants. District organizations of the Union were launched in Petrograd. Gradually, the Union turned into an All-Russian center for uniting anti-Bolshevik forces under the slogan “All Power to the Constituent Assembly!”. It received telegrams from emerging local organizations.

The Union conducted propaganda among the soldiers of the Preobrazhensky and Semenovsky regiments, and the 5th Armored Division in order to create armed support for the Constituent Assembly. The All-Russian Extraordinary Commission for Combating Counter-Revolution and Sabotage (VChK) established surveillance of the Union’s headquarters and repeatedly conducted searches. On December 16, 14 Union activists were arrested.

On January 5, 1918, on the Constituent Assembly opening day, the Union organized a demonstration, which was attended by up to 60 thousand people. Initially, the military department of the Union, together with the military commission of the Socialist Revolutionary Party prepared an armed rebellion, but the Socialist Revolutionary leadership decided to abandon the plan. According to official data, 12 people were killed during clashes between demonstrators and detachments of Red Guards and sailors.

After the dissolution of the Constituent Assembly on the night of January 6 to 7, 1918, the Union ceased its activities. The last major event of the organization was the funeral of those who were killed on January 9, 1918 (the “Bloody Sunday” anniversary). The investigative commission under the Petrograd Soviet of Workers’ and Soldiers’ Deputies tried to investigate the actions of the VChK regarding the Union, but sufficient material was not collected to launch the trial. Previously arrested Union leaders were released in the second half of January 1918. The publication of leaflets lasted until March 1918.