Deep divisions over support for Mrs Merkel in a new coalition government led to the resignation of SPD leader Martin Schulz this week and the favourite to replace him has told supporters she is up for the fight.
Andrea Nahles backs renewing the alliance but promised to help the party reinvent itself by fighting for its policies in a government with Mrs Merkel’s Christian Democrats (CDU).
She told a rally in Schwerte, a town in the SPD’s traditional heartland of North Rhine-Westphalia: “We will not do a runner in this government.
“We will make our own policy proposals. We will consciously stand up to Mrs Merkel.”
Her fiery speech signalled the next marriage between the two largest parties would not be as harmonious as the previous two grand coalitions during Mrs Merkel’s 12 years in power.
Ms Nahles said: “She may be the most powerful woman in the world but the Twilight of the Gods started a long time ago. “She has been put down for the count by her own party. The chance to renew stands before us.”
Ms Nahles, who hopes to be elected as SPD leader at an extraordinary party congress on April 22, urged the SPD’s 464,000 members to approve in a postal ballot starting on February 20 a four-year government programme that party leaders clinched last week.
The results of the vote will be announced on March 4 and ff the deal is rejected, Germany will probably have to hold a new election.
Many within the SPD have grave misgivings about sharing power with Mrs Merkel again and believe the party should rebuild in opposition after suffering its worst result in last September’s election since Germany became a federal republic in 1949.
Party leaders, facing a slump in opinion polls, have their work cut out trying to convince the membership to back the deal which has seen the party slide into rapid disarray with even Mr Schulz’s departure, which was intended to steady the ship, sparking a bitter internal row.
But Mrs Merkel is also facing discontent with the deal among conservatives who feel her concessions to the SPD went too far.
Earlier this week she rejected suggestions that rising criticism among conservatives of her decision to cede the finance ministry to her rivals was a sign that her authority was on the wane.