While studying Chemistry at the University of Santiago de Compostela, he joined the Communist Party of Spain (PCE) which was still outlawed by Francisco Franco.
Inspired by the May 68 protests in Paris, he took part in clandestine political activity and was imprisoned three times.
[1] In November 1983, he was replaced as party leader by Julio Pérez de la Fuente, who stood on a more pro-Soviet platform than Guerreiro's reformist Eurocommunism.
[2] Having lost his seat in the 1985 election, in 1987 Guerreiro founded Galicia's United Left, the regional branch of the PCE-led coalition of the same name.
In 1997, he wished to contest the Galician elections in alliance with the Spanish Socialist Workers' Party (PSOE), which was rejected by the national United Left.