In the 2009 federal election, Bas was elected in the constituency of Duisburg I for the SPD as an MP in the 17th German Bundestag. In the 2013 federal election and in the 2017 federal election, she was able to defend her direct mandate. In the 17th Bundestag she was a full member of the Committee on Health, to which she continued to be a deputy member since in the 18th Bundestag. Since 2014, she has been a member of the parliament’s Council of Elders, which – among other duties – determines daily legislative agenda items and assigns committee chairpersons based on party representation. She is also a deputy member of the Gemeinsamer Ausschuss ("Joint Committee") of the Bundesrat and Bundestag.