Keul has been a member of the Green Party since 2006.[2]
Keul has been a member of the German Bundestag since the 2013 elections, representing the Nienburg II – Schaumburg constituency. In parliament, she served on the Committee on Legal Affairs and the Committee on Defense from 2013 until 2021. During that time, she served as her parliamentary group's spokesperson on legal affairs. From 2014 until 2017, she was also part of the parliament's Council of Elders, which – among other duties – determines daily legislative agenda items and assigns committee chairpersons based on party representation. Following the 2017 elections, she joined the Subcommittee on Disarmament, Arms Control and Non-Proliferation.[citation needed]
In 2014, Keul – alongside fellow Green Party parliamentarians Claudia Roth and Hans-Christian Ströbele – lodged a complaint before the Federal Constitutional Court of Germany, arguing that it was unconstitutional for the government to keep the Bundestag in the dark about planned arms deals because it prevented the parliament from fulfilling its role of keeping the government in check. The court ruled that while the government did not have to disclose information about planned defense exports, it did have an obligation to provide the Bundestag with details, on request, once specific arms deals had been approved.[13]
In 2016, Keul and Volker Beck submitted a compensatory draft law to all parliamentary groups in the German Parliament, urging them to remove Paragraph 175 in the penal code, which criminalized homosexual acts.[14]