She joined the German Social Democratic Party (SPD) in 1965 and became a member of the Bundestag in 1972. She held several party offices in the 1980s and '90s, serving as deputy party chairman from 1988-97. From 1994-98 she was chairwoman of the "Rechtspolitik" working group and legal adviser to the SPD parliamentary group. In 1993, the SPD nominated her to fill the vacancy of vice-president of the Federal Constitutional Court, but after conservative parliamentary groups blocked the nomination for nine months as being "too political" she abandoned this career step in favor of Jutta Limbach.
On September 18, 2002, four days before Schröder's re-election, she attended a meeting at a restaurant in Derendingen (near Tübingen) with about 30 trade unionists from two local factories (the topic was "Globalization and Labor"). Däubler-Gmelin, who has long been known for her outspokenness, later claimed to have been unaware that a reporter from local newspaper Schwäbisches Tagblatt was present, insisting that she regarded the event as an internal meeting. After discussion had turned to the Iraq crisis, she made a remark that U.S. president Bush was preparing a war to detract from domestic problems such as the economic crisis at the time, that this was a popular political strategy used by many - she mentioned Margaret Thatcher and the Falklands War - and known since Adolf Hitler. When some participants showed disagreement, she added immediately that this was not meant to liken Bush to Hitler as a person, but rather to compare their methods. She also described the U.S. legal system as "lousy".
This was the version published by Schwäbisches Tagblatt (a paper widely regarded as liberal to leftist and respected for its journalistic quality), which states that Däubler-Gmelin herself had confirmed the wording of the report, as well as several present at the meeting. Another account of the meeting claims that the Hitler comparison originated from a participant and that Däubler-Gmelin had merely agreed that Hitler had used these tactics, too.
From 2002 to 2005 she was chairman of the Bundestag's Committee on Consumer Protection and Agriculture, and since 2005 she chairs the Committee for Human Rights and Humanitarian Aid.
Däubler-Gmelin is a member of several charitable and non-profit organizations. Her husband is Wolfgang Däubler, one of the most prominent experts on German labor law. They were married in 1969 and have two children.