An independence and peasant activist before Poland regained an independence in 1918, Rataj became Member of Sejm in 1918. He was elected Speaker in the first regular term and held that post from 1922 to 1928. Previously, he was Minister for Religious Affairs in Wincenty Witos cabinet.
He served as Acting President twice: from 16 December to 22 December 1922 and from 15 May to June 4 1926. For the first time he served between assassination of Poland's first President Gabriel Narutowicz and election of his successor Stanisław Wojciechowski. For the second, between Wojciechowski resignation after a May coup and election of Ignacy Mościcki.