He was defeated for re-election in 1946 by Republican Charles J. Kersten, and went to work as director of political education for the Upholsterers Union. During this period Biemiller joined many other former Socialists and Progressives in helping to found Americans for Democratic Action. He cooperated with Hubert Humphrey in successfully calling for a strong civil rights plank at the 1948 Democratic National Convention, to which he was a delegate and chair of the platform committee which produced the plank.