How did the Social Democratic Party of Germany oppose Adolf Hitler during the Third Reich?

During the Third Reich, the Social Democratic Party of Germany (SPD) maintained its opposition to Adolf Hitler through two primary channels:

• Exile Organizations: The party established and maintained organizations in Britain and the United States to continue its political work and opposition from abroad.1
• Underground Resistance: An underground network operated within Germany throughout the Nazi era, managing to survive and remain relatively intact despite the regime's intense persecution.1

Following the Nazi rise to power in 1933, the SPD was officially outlawed, and many of its prominent members were arrested and imprisoned in concentration camps. Because of these efforts, the SPD emerged after the fall of the Third Reich in 1945 as the only surviving political party from the Weimar period with an unblemished record of opposition to Nazism.12