He was one of the founders of the Partido Comunista de España (PCE, Communist Party of Spain).

[4] During the dictatorship of Miguel Primo de Rivera (1923–30) he was arrested several times and spent much of this period in prison.

In 1935 he was appointed to the central committee of the newly formed Euskadiko Partidu Komunista (EPK, Basque Communist Party).

[1] At the time of the 18 July 1936 military revolt that began the Spanish Civil War (1936–39) Carro was on a propaganda tour.

When the resistance was crushed he retreated to the mountains of Maceda where he joined in guerrilla actions against the forces of General Francisco Franco.

[2] Carro went into hiding in the north of Portugal to avoid the police of Prime Minister António de Oliveira Salazar, who was hostile to communists.

[2] After the war ended he moved to France, where he represented the EPK as Minister of Public Works in the Basque Government in exile headed by Jose Antonio Aguirre from 1946 to 1948.

[2] On 19 May 1948 Aguirre wrote to Carro explaining that the PCE should retire from the government due to their incompatibility with the socialists, although he would retain normal and friendly relations with the communists.