"For now we want to convince the SPD's members to vote in favour of accepting the coalition agreement", said Hamburg's mayor, Olaf Scholz, who has been mooted as a contender for the finance ministry post. The party's youth wing is pushing the Social Democrats to go into opposition, and leadership questions mired its top officials.
Facing growing unrest from within her long-obedient CDU, party chief Merkel chose to defend the coalition deal, which must still be approved in an SPD party vote.
"I will lobby in favor of entering into a grand coalition", Nahles told reporters in Berlin, referring to the SPD's alliance with Merkel's Christian Democratic-led bloc that governed Germany for the last four years.
Merkel said on Sunday that if SPD members rejected the coalition agreement, Germany would probably hold a new election.
Contrary to Schulz, an outsider for many SPD members, Nahles is deeply entrenched in the party.
Andrea Nahles poised to take over as as SPD leader after resignation of Martin Schulz.
Nahles said she would start campaigning at the weekend for members to vote "yes" to a coalition with Merkel, who has led the European Union's most populous country and economic powerhouse since 2005.
Paul Ziemiak, leader of the conservatives' youth wing, welcomed Merkel's readiness to set out her picks for ministerial posts before a CDU party conference on February 26, but lamented the decision to give up the powerful finance ministry to the SPD.More news: Pennsylvania's congressional map drawn, but still not passed
The development followed Schulz's announcement Friday that he was abandoning plans to become foreign minister in the new government.
Germany has been without a formal government since the September 24 election and investors are anxious about a delay in policymaking, both at home and in the EU.
Scholz played up the SPD's achievements in coalition negotiations with Merkel's Christian Democrats (CDU) and her Bavarian allies, at which the centre-left party won the powerful finance ministry to the frustration of many in the CDU.
An INSA poll published on Tuesday showed the SPD at a record low of 16.5 percent, only 1.5 percentage points ahead of the far-right Alternative for Germany (AfD).
And Mrs May will round-off the process in about three weeks' time with a pitch on what kind of overall relationship the United Kingdom wants with the European Union after we leave.
Ralf Stegner, one of the SPD's deputy leaders, called for an end to "indiscipline" in the party.
The SPD originally wanted to reinvent itself in opposition but reconsidered when Merkel's attempt to form an alliance with two smaller parties failed late past year.