Germany’s embattled coalition parties were holding separate crisis talks on Monday after the resignation of the leader of the centre-left SPD plunged the government into turmoil.
Andrea Nahles said she was stepping down following her party’s poor performance in the European elections.
Her resignation has raised concerns that Chancellor Angela Merkel’s coalition government could collapse.
But Mrs Merkel’s party has insisted it is ready for the coalition to carry on.
Rolf Mutzenich, deputy chairman of the SPD, put himself forward to serve as the party’s interim leader after an initial meeting on Sunday.
But three caretaker leaders are set to take over until Ms Nahles’ successor is elected: Manuela Schwesig is state premier in Mecklenburg-Vorpommern, Malu Dreyer is premier in Rhineland-Palatinate and Thorsten Schäfer-Gümbel is party leader in the Hesse.
Germany’s dpa news agency says the interim joint-leadership has been proposed to the party’s executive board.
Ms Nahles’ long-term replacement is yet to be determined, leaving the coalition hanging in the balance while further talks take place.
The 48-year-old’s centre-left party came third, behind Mrs Merkel’s Christian Democrats (CDU) and the Greens, in the 26 May elections.
The poor result has caused disquiet in the party, whose left-wing members have criticised Ms Nahles for remaining in the coalition.
Ms Nahles became SPD leader in April 2018, replacing Martin Schulz who had also resigned because of poor electoral performance. She had been expected to run for the position again and her resignation took analysts by surprise.
The coalition between the CDU and the SPD is due to last until federal elections in 2021, but correspondents say Ms Nahles’s resignation could lead to the SPD leaving, triggering a snap poll.
Mrs Merkel herself plans to step down as chancellor in 2021, having already resigned as CDU leader at the end of last year.
Mrs Merkel’s Christian Democrats (CDU) were also meeting to discuss the crisis as the junior partner in the coalition considered its next step.
CDU leader Annegret Kramp-Karrenbauer told reporters that the party was willing to continue sharing power with the Social Democrats. “We want to do justice to the government mandate,” she said.
Several senior party figures in the Social Democrats, including Vice-Chancellor Olaf Scholz and Lower Saxony’s Prime Minister, Stephan Weil, have ruled themselves out of the leadership race.
Former SDP leader Thomas Oppermann said it could take “one, two months” to choose Ms Nahles’ replacement.
He said the political instability was “not a good overall situation” and warned his party against “waiting for further defeats”, alluding to the forthcoming elections in three East German states in September and October.
Polls suggest the SDP may suffer losses in those elections. In a Forsa survey, the SPD dropped by five percentage points to 12%, its lowest-ever score on a national level.
Ms Nahles said on Sunday she would stand down as SPD leader and as head of its parliamentary group.
“The discussions within the parliamentary faction and feedback from within the party have shown me that I no longer have the necessary support to carry out my duties,” she said in a a statement.
If the SPD were to leave the coalition, the fall of the government would be likely to trigger fresh elections.
Olaf Scholz told Tagesspiegel newspaper he had ruled out entering another such coalition.
“Three grand coalitions in a row would not do democracy in Germany any good,” he said in the interview before Ms Nahles announced her resignation.
The latest crisis come days after Mrs Merkel dismissed reports of a rift with Mrs Kramp-Karrenbauer.
Two unidentified officials quoted in a Bloomberg article had said Mrs Merkel believed her successor was not up to the job. But the German chancellor dismissed the claims as nonsense.