 In the negotiations to form a "Grand Coalition" of Chancellor Angela Merkel's Christian Democrats (CDU together with the Bavarian CSU) and the SPD following the 2013 federal elections, she was part of the SPD delegation in the working group on health policy, led by Jens Spahn and Karl Lauterbach.Appointed by Federal Minister of Health Hermann Gröhe, she served as member of an expert commission on the reform of Germany’s hospital care from 2015 until 2017.Mattheis was (together with her running mate Dierk Hirschel) a candidate for the 2019 Social Democratic Party of Germany leadership election; however, she withdrew her candidacy shortly before the vote.