In February 1917 he won ALP pre-selection for a by-election in the seat of Sturt. The by-election had been caused by the resignation of the previous member John Cann who had accepted the position of Railways Commissioner. Brookfield won the seat with 54% of the primary vote and increased this to 57% at the general election that was held one month later. In parliament he became a leading left-wing advocate and expressed sympathy for the Industrial Workers of the World and the Bolshevik Revolution. His radicalism led him into conflict with the rest of the ALP caucus and he resigned from it in July 1919. He was not readmitted to the party when he attempted to retract his resignation. He later then joined the Industrial Socialist Labor Party.