The SPD leadership and many other members are in favour of the coalition between the party and Angela Merkel's conservatives. But the youth wing of the party and left-wingers are fighting for a "No Grand Coalition" vote.
Members of Germany's Social Democrats (SPD) start voting on whether their party should enter a new coalition with Angela Merkel's conservatives in a postal ballot on Tuesday which could scupper the chancellor's fourth term.
If the SPD's nearly half a million members reject the deal, a new election or a minority government in Europe's biggest economy are likely. Both would be a first for post-war Germany, which has been without a formal government for nearly five months.
The result of the vote, which runs to March 2, is wide open and will be announced on March 4. Polls indicate a majority of SPD supporters back the deal, but in the absence of any internal surveys it is unclear whether its members feel the same way.
The party leadership and many in its senior ranks want the coalition to go ahead, but there is also strong opposition to the deal within the SPD. The party is at record lows in opinion polls and embroiled in a messy leadership switch after a personal row between former leader Martin Schulz and Foreign Minister Sigmar Gabriel.
Opponents to the deal blame their party's role in a "grand coalition" from 2013 to 2017 for their worst performance since World War Two in the September 24 election.
In January, a narrow majority of SPD delegates at a special congress backed entering formal coalition talks with Merkel.
SPD leaders – Olaf Scholz, the interim leader, and Andrea Nahles who is expected to take over in April – are touring the country to convince members to approve the deal.
They are stressing how they got Merkel to concede the two top ministries, finance and foreign, to the SPD and on Sunday, a poll showed that leading SPD mayors favoured joining a coalition.
Youth wing's stance
However, the substance of the coalition deal falls short for many in several areas, including health and migrant policy.
The vibrant Jusos youth wing of the party and left-wingers are on their own tour, fighting for a "No Grand Coalition" vote and their message has resonated with many.
A campaign to get people to join the party and vote against a coalition succeeded in boosting membership by about 24,000 to almost 464,000.
It is unclear which side the latest slump in the polls will help. An INSA poll released on Monday showed the SPD had fallen to 15.5 percent, below the far-right Alternative for Germany (AfD), on 16 percent, for the first time.
One of the SPD's deputy leaders, Thorsten Schaefer-Guembel, told Germany's SWR radio that the SPD's situation was "serious."
"But if we remember our strengths, sharpen the substance (of our policies) and end the debate over personalities, then we are, I think, on a good path," he said.