Schulz said last week he would quit to allow the party to regroup and he recommended Nahles as leader.
Ms Nahles, the 47-year-old former employment minister, remains favourite to succeed Mr Schulz and become the first woman to lead the party but Simone Lang, the relatively unknown mayor of Flensburg, a small city near the Danish border, has announced she will stand and force a vote.
The head of Germany's Social Democrats (SPD), Martin Schulz, has resigned to ease preparations for a coalition government with Chancellor Angela Merkel's conservatives.
As recently as Monday, SPD secretary general Lars Klingbeil reiterated a long-standing demand for a change in party statutes which would require a binding membership vote on the election of any leader rather than their selection by the party's elite leadership circle. Schulz said after the deal was announced that he would step down following the members' vote - the results of which are due to be announced on March 4 - to take the job of foreign minister, but later dropped that plan amid controversy. The party's youth wing is already campaigning for a no-vote as it fears that another grand coalition with Mrs Merkel's conservatives will further erode the party's identity. If members reject the coalition pact, a new German election looks the most likely option.
Ralf Stegner, the regional SPD leader in Schleswig-Holstein, said: "Each of us would be well advised to put the interest of the party and the country before their own ambitions".
He said he hoped the party could "regain its former strength" under Nahles' leadership and as part of the German government - if members agreed to that in the upcoming ballot.
Malu Dreyer, regional prime minister of Rhineland-Palatinate, said: "The SPD can not remain leaderless".
In a cartoon on Tuesday, the Sueddeutsche Zeitung daily showed Nahles with a whip riding an SPD snail. German media cited a letter by Lange to the SPD leadership in which she called on the party to open itself up to a truly democratic debate over its future orientation. The former European Parliament president said last Wednesday that he planned to become Germany's next foreign minister - a risky move given that, after the election, he had explicitly ruled out entering Merkel's next cabinet. "I am surprised that Andrea Nahles wants to take over the leadership immediately, if only in the interim", Harald Baumann-Hasske, Chairman of the Working Group Social Democratic Lawyers, told the newspaper "Welt".
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