He became a member of the National Executive of the Socialist Youth of Spain (Juventudes Socialistas de España, JSE), and editor of Claridad.

[2] Writing in the 21 December 1933 issue of Rundschau, the German-language journal of the Communist International, Melchor said the Communist Party of Spain (Partido Comunista de España, PCE) organizations had tried to take leadership of the mass revolts that took place in Spain that month, and in some cases succeeded.

[1] He visited Moscow in February–March 1936 with Santiago Carrillo, José Laín Entralgo, Trifón Medrano Lherba, Felipe Muñoz Arconada and Juan Ambou.

The JSU committed itself to being a "new style" Popular Front youth movement as described by the Communist International.

[6] Other key posts in Carrillo's department were held by Luis Rodríguez Cuesta (secretary of the Council), Segundo Serrano Poncela of the General Directorate of Security (Dirección General de Seguridad, DGS), Fernando Claudín Pontes (Press Cabinet) and Alfredo Cabello (Emisión Radiofónica).

The police squads formed by trade unions or political parties were replaced by full-time security forces that the communists dominated.

[8] Melchor transferred from the Spanish Socialist Workers' Party (Partido Socialista Obrero Español, PSOE) to the PCE in November 1936.

[9] The PCE opposed socialization of industry, which the anarcho-syndicalist Confederación Nacional del Trabajo (CNT) supported.

"[11] Under the government of Juan Negrín the Sub-secretariat of Propaganda was headed by Leonard Martín Echevarría of the Republican Left.