 From 2005 until 2007, she was part of the parliamentary group’s leadership around chairman Peter Struck.She has been part of internal working groups on health (since 2002), migration and consumer protection (since 2019).In 2009, Mattheis came in second only after Nils Schmid in an internal party vote on the leadership of the SPD in Baden-Württemberg; she subsequently became one of his four deputies.