He joined the Social Democratic Party of Germany (SPD) in 1910, where he was a member of the left wing and volunteered in several capacities.
During this period, Westermann sympathized with the Spartacus League and the Independent Social Democratic Party of Germany.
[1] In 1925, Westermann was briefly expelled from the KPD after he advocated a waiver of Ernst Thälmann's candidacy in favor of Social Democrat Otto Braun, a tactical move meant to thwart a victory by Paul von Hindenburg in the second round of the German presidential election in 1925.
[1] After the Nazis seized power in 1933, Westermann's group began working underground, focusing on dock and shipyard workers and employees.
On his release, made contact with other Conciliator groups both within and outside the KPD, such as the Committee for Proletarian Unity, founded by Eduard Wald.