Two others – both on the party’s left – are against, including second place in tonight’s vote, Norbert Walter-Borjans from Germany’s most populous state, North Rhine-Westphalia.
Few of the candidates are household names and analysts say the protracted leadership campaign has merely highlighted the lack of quality in the top ranks of the party.
Any new leader will have to unite a party that has been bitterly divided between leftists and centrists since former SPD Chancellor Gerhard Schroeder’s welfare and labour market reforms 15 years ago.
Facing a surge in support for the Greens on the left and with the far-right Alternative for Germany (AfD) collecting protest votes, the decline of the pro-European party proud of chancellors including Willy Brandt has been dramatic.