Following the events of 25 July (Benito Mussolini's deposition) in 1943, he left the army and entered both the Italian resistance movement and the National Liberation Committee as a member of the Action Party (Partito d'Azione; PdA).

Although a committed partisan, by the end of the war he harboured a profound distrust of the Italian Communist Party (Partito Comunista Italiano; PCI) for its role in concealing the death of Mussolini.

Known for his strong personality and short temper, Bucalossi was a sincere believer in small government (despite his party's statist, social-democratic philosophy), similar to that of an American fiscal conservative.

[2] He introduced balanced budget and austerity policies, which cut the city administration's spending drastically, and opposed the creation of regional councils, sarcastically dubbing them "parlamentini" ("small parliaments").

By this stage, he had become a leading critic of the Historic Compromise between the PCI and the Christian Democrats (Democrazia Cristiana; DC), arguing that the experience of having Communists in the governmental majority had brought about a decline in the wellbeing of the country.