In January 1919, Müller was elected to the National Assembly. In February 1919, Ebert became president of Germany and appointed Philipp Scheidemann as "Ministerpräsident" (head of government). These two had been the joint chairmen of the SPD and now replacements had to be found. Müller and Otto Wels were elected with 373 and 291 out of 376 votes, respectively. Wels focused on internal leadership and organization, whilst Müller was the external representative of the party. In 1919 and 1920–28, Müller was also leader of the parliamentary fraction ("Fraktionsvorsitzender ") in the National Assembly and then the Reichstag. He was nominated as the chairman of the Reichstags' Committee on Foreign Affairs. After 1920, he was a candidate for the Reichstag for Franconia and changed his name to Müller-Franken, to distinguish himself from other members named Müller.