On 13 February 1919, Preuß became "Reichsinnenminister" (Interior Minister) in the first elected government of the republic under "Ministerpräsident" Philipp Scheidemann (SPD). Preuß opposed vehemently the Triple Entente prohibition of the incorporation of German Austria into Germany as a contradiction of the Wilsonian principle of self-determination of peoples. The government resigned on 20 June 1919 in protest against the Treaty of Versailles. It was followed by the government of Gustav Bauer (SPD), who appointed Preuß "Reichskommissar für Verfassungsfragen" (commissioner for constitutional issues). On 14 August 1919, the Weimar Constitution came into force.