He then joined the Communist Party of Argentina (PCA) in 1920, being elected to its Central Committee and Politburo (on which he would serve until his death in 1970).

Under the alias Luis Medina, he presented a paper to the PCE's fourth congress in March 1932, proclaiming that the new Second Republic was supported by monarchism on the right and "social fascists" (such as the members of the Spanish Socialist Workers' Party) on the left.

He further predicted that the administration of President Manuel Azaña would quickly become a "clear-cut fascist dictatorship" and stressed the need for Spanish communists to create revolutionary committees and establish Soviets in order to stem the counterrevolution.

[3] With the outbreak of the Spanish Civil War in 1936, Codovilla remained in Madrid advising the PCE, but has been regarded as generally less effective than his Barcelona-based counterpart Ernő Gerő.

[5] By the summer of 1937 reports to Moscow by fellow Comintern advisor Palmiro Togliatti were critical of both the state of the PCE leadership and Codovilla.