A member of the moderates faction, he was strongly against conscription in the First World War, and in 1916 became a founding member of the Industrial Vigilance Committee, which was responsible later that year for the expulsion from the Labor Party of conscription advocates such as prime minister Billy Hughes and premier William Holman.

In the same year, on 26 April 1919, Carlo married Myra Hurley at St Brigid's church in Marrickville.

Labor regained government in 1930, only to lose it after controversies of 1932, culminating in the governor withdrawing Lang's commission.

Lazzarini maintained his opposition to Lang and his dictatorial style, one of the few in the New South Wales caucus to openly do so.

Lazzarini joined the Industrial Labor Party in 1938, and continued to apply pressure to Lang, who was finally ousted in August 1939.