Simone Lange, the Social Democratic mayor of the northern German town of Flensburg, is set to challenge Andrea Nahles to replace current SPD leader Martin Schulz at the head of the party.

The surprise announcement comes amid growing push-back within the SPD against senior officials who had planned to hand over leadership to Nahles, who currently heads the SPD's parliamentary group.

Read more: Who is Andrea Nahles, the potential new SPD leader?

What happened:

• Simone Lange wrote a letter to the SPD executive board announcing her candidacy for the top position, according to reports on Tuesday from regional news portal shz.de and public broadcaster NDR (German-language).
• Lange said she wanted to give SPD members a voice at a time when many had "the feeling of being powerless compared to those who make decisions in Berlin without consulting the party's grassroots."
• Martin Schulz, the SPD's current leader, announced on Wednesday his intention to step down and pass on the position to Andrea Nahles after party members vote on a new coalition deal with Chancellor Angela Merkel's conservatives in March.

Read more: Martin Schulz, leader of Germany's Social Democrats in profile

Provisional appointment: The SPD's executive board and steering committee are able to appoint Nahles as provisional leader when they meet later on Tuesday. Around 600 SPD delegates would have to approve the provisional appointment within three months.

Read more: The young SPD politician who could topple Angela Merkel and Martin Schulz

Internal pushback: Some party factions have criticized the prospect of provisionally appointing Nahles, who is not an elected deputy, as leader. Under normal party procedure, one of the SPD's six elected deputies would take over temporarily after a leader resigns. The regional SPD branches in Berlin and the northern state of Schleswig-Holsten, where Flensburg is located, have reportedly called on the party to respect this policy.