Ms Nahles was named by Mr Schulz as his chosen successor last week. At the time he was still hoping to become foreign minister by giving up the leadership, a plan he had to abandon in the face of massive opposition.
Ms Nahles is still backed by most of the current leadership, and several grandees have stepped in to try to quell the debate.
“The SPD cannot remain leaderless. It was right when Martin Schulz proposed Andrea Nahles take over the interim leadership,” Malu Dreyer, the influential regional prime minister of Rhineland-Palatinate said.
“Each of us would be well advised to put the interest of the party and the country before their own ambitions. No more indiscipline in the SPD,” said Ralf Stegner, the regional SPD leader in Schleswig-Holstein.
But three regional associations led by the powerful Berlin local party have objected to plans to install Ms Nahles as interim leader. They say it is against party rules and one of the current deputies should step in until members can vote on a new leader.
A formal election will now have to be held after Ms Lange declared her candidacy yesterday. Virtually unknown outside her native Flensburg, Ms Lange said she was standing in order to force a contest.