In 1910, Karl Kilbom moved to Halmstad to do work for the Social Democratic party there. Within the party, Kilbom sided with the Left Opposition led by Zeth Höglund against the reformist party leader Hjalmar Branting. In 1917 the party split in two and Kilbom joined its Left-leaning faction, which supported the Bolsheviks in Russia and was called the "Social Democratic Left Party of Sweden". It soon evolved into the Communist Party of Sweden. Already in 1915, Karl Kilbom had been made one of the main Swedish contacts with the Russian Bolsheviks and worked closely with Bukharin who lived in Sweden during the war.