Armstrong was known in this period as a leading figure in the SPC's "impossibilist" wing, opposing any cooperation with moderate labour.

In the 1914 provincial election, Armstrong ran in Winnipeg Centre "B" against Fred Dixon, an independent candidate supported by both the Liberals and the Labour Representation Committee, a successor to the MLP.

The Winnipeg General Strike of 1919 brought Armstrong and the SPC into cooperation with the city's labour movement.

Along with other prominent labour organizers in the city, Armstrong was brought to trial after the strike's suppression on charges of seditious conspiracy.

Armstrong, previously an opponent of "popular front" strategies, became the SPC's star candidate on Winnipeg's united labour list for the 1920 provincial election.

During the 1922 provincial election, Armstrong was frequently heckled by Communist candidates who accused him of being a "sell out" to the social gospellers in the mainstream labour movement.

Armstrong ran for the Manitoba legislature again in the 1932 provincial election as the candidate of the Socialist Party of Canada (WSM).