Since the 2021 elections, Müller has been serving as one of his parliamentary group's deputy chairs, under the leadership of successive chairs Ralph Brinkhaus (2021–2022) and Friedrich Merz (2022–present). In this capacity, he oversees the group’s legislative activities on health policy, sports and petitions.[3] He was appointed to the role after he won an internal vote against Christian Hirte.[4] Within the CDU/CSU parliamentary group, he has also been leading the group of CDU parliamentarians from Saxony-Anhalt since 2021.[5]