The proposal to return to the Republican-Socialist alliance of 1931-33 was suggested in 1935, during the Radical Republican Party-CEDA government, on the initiative of Manuel Azaña, the leader of the Republican Left Party. Azaña had become, after his unjust arrest during the October Revolution of 1934, a "political martyr" and symbol for the left, and the entry of more CEDA ministers into the Government in 1935 caused him to undertake a tour of Spain, including mass meetings in order to achieve a "Republican intelligence" that would restore the regime to its democratic values.
In April 1935, Azaña had reached a pact of "Republican Conjunction" between his own party (a merger of Republican Action, part of the Radical Socialist Republican Party and the Autonomous Galician Republican Organization), Diego Martínez Barrio's Republican Union and the National Republican Party led by Felipe Sánchez-Román. In the middle of 1935 Azaña offered to form an electoral coalition with the PSOE based on the agreement of the combined forces of the republican left.
Whilst the section of the PSOE led by Indalecio Prieto defended the agreement, that of Francisco Largo Caballero was reluctant. In order the protect the "worker's" aspect of the coalition, Largo Caballero insisted on the inclusion of the Communist Party of Spain, an act which motivated the departure from the Republican Conjunction of Sánchez-Román's National Republican Party. The PCE, for its part, had changed its position on the socialists (who hitherto had been considered "enemies" of the revolution) after the VII Congress of the Third International in Moscow, at which Stalin had launched the new slogan of forming "anti-fascist fronts". The signing of the electoral coalition pact between the left republicans and the socialists took place on the 15th January 1935. The signature of the PSO was also given on behalf of the PCE and other worker's organisations (the Syndicalist Party and the POUM).