 He was elected to the Legislative Assembly of Quebec as the Quebec Liberal Party MLA for Kamouraska in the 1912 provincial election and was re-elected in 1916 and 1919.He resigned his provincial seat on March 31, 1920 in order to enter federal politics when his law partner and Kamouraska MP Ernest Lapointe resigned his seat in order to run in Quebec East which had become vacant due to the death of Sir Wilfrid Laurier.Stein was elected to the House of Commons of Canada in a 1920 by-election and was re-elected in the 1921 federal general election as the federal Liberal Party MP for the federal riding of Kamouraska.