Early beginnings

Haßelmann became a member of the Green Party in 1994. From 2000 until 2006, she served – alongside Frithjof Schmidt – as co-chair of the Green Party in North Rhine-Westphalia, the party's largest chapter.^[3] During that period, her party was in a coalition government with the Social Democratic Party under Minister-President Wolfgang Clement.

Member of the German Parliament, 2005–present

Haßelmann has been a member of the German Bundestag since the 2005 federal election, representing Bielefeld. From 2005 until 2017, she served on the Finance Committee. In 2009, she also joined the Council of Elders, which – among other duties – determines daily legislative agenda items and assigns committee chairpersons based on party representation. From 2017, she served on the Committee on the Scrutiny of Elections, Immunity and the Rules of Procedure as well as on the Committee on the Election of Judges (Wahlausschuss), which is in charge of appointing judges to the Federal Constitutional Court of Germany. She is also a member of the Vermittlungsausschuss [de] of Bundestag and Bundesrat.^[4]

Within her parliamentary group, Haßelmann served as Chief Whip from 2013 until 2021, under the leadership of the group's co-chairs Katrin Göring-Eckardt and Anton Hofreiter. In the – unsuccessful – negotiations to form a coalition government with the Christian Democrats – both the Christian Democratic Union (CDU) and the Christian Social Union in Bavaria (CSU) – and the Free Democratic Party following the 2017 elections, she was part of her party's delegation.

From 2018, Haßelmann was part of a cross-party working group on a reform of Germany's electoral system, chaired by Wolfgang Schäuble.^[5]