From 1983 to 1984, he served as the head of the European election office in the SPD party headquarters, where he gained significant experience in the political arena under the leadership of Willy Brandt.

From 2010 until 2017, he served as deputy chairman of the SPD parliamentary group under the leadership of successive chairpersons Frank-Walter Steinmeier (2010–2013), Thomas Oppermann (2013–2017) and Andrea Nahles (2017).

[6] In October 2024, Schäfer announced that he would not stand in the 2025 federal elections but instead resign from active politics by the end of the parliamentary term.

[8] Following Andrea Nahles’ resignation as chairwoman of the SPD in 2019, Schäfer proposed Manuela Schwesig and Stefan Weil as new leaders.

[9] Ahead of the party’s 2019 leadership election, he publicly endorsed Olaf Scholz and Franziska Giffey as potential chairpersons.