A rebellion is also growing in the ranks of Merkel's conservative CDU, with former allies arguing that she had ceded too much power to the left-leaning SPD in order to rule for a further four years as the head of a coalition government.
German Social Democrat leader Martin Schulz confirmed on Friday he would not take up the position of foreign minister or serve in the next government, seeking to end a party row over his role and shore up members" support for a new "grand coalition'.
The rare public broadside at his comrades came after SPD and Merkel's conservative CDU/CSU bloc announced a hard-fought coalition deal on Wednesday that could see Schulz claim the Foreign Ministry.
The perception among Social Democratic Party activists that party chief Schulz had fashioned himself a life raft from the wreckage of an election disaster threatened chances the rank and file would green-light a controversial coalition deal sealed just two days ago. He was officially elected by a party congress with 100 percent of the vote.More news: No, we don't need a tank parade
Many young and leftist members are against the renewed so-called Grand Coalition between the SPD and the Union.
Prospective German Foreign Minister Martin Schulz has announced that he will not take up a ministerial post in the next coalition government.
Last September, the Social Democrats suffered their worst election result in decades, but remained the second-largest party in parliament.More news: Olympics Opening Ceremony: Who is U.S. flag bearer Erin Hamlin?
"I know it wasn't an easy decision for him", Nahles told reporters in her home village of Weiler in western Germany, saying Schulz deserved respect and recognition for stepping aside so as not to jeopardise the party vote on the coalition.
Many in the SPD fear that re-entering a coalition with Mrs Merkel's Christian Democrats (CDU) and the Bavarian CSU could damage the party in the long term.
German Chancellor and leader of the Christian Democratic Union (CDU) Angela Merkel arrives for the coalition negotiations at CDU headquarters on Tuesday.More news: Cavs' Actual Return for Kyrie Irving Revealed After Isaiah Thomas, Other Trades