Counting began on Saturday in Berlin to see if members of Germany's SPD had approved a decision to join a coalition government led by the German chancellor Angela Merkel.
Germany will learn on Sunday whether the Social Democrat (SPD) party's 464,000 members have voted in favour of a coalition with Chancellor Angela Merkel's conservatives, opening the way to a new government for Europe's economic powerhouse.
Five months after an inconclusive election, and after the failure of Merkel's first attempt to form a government with two smaller parties, the long-serving chancellor's best hope of securing a fourth term lies with the SPD.
"We already know there will have been a very, very high level of participation," said the SPD's acting leader Olaf Scholz as he arrived at the party's Berlin headquarters where volunteers will count the postal ballots overnight.
His colleague Andrea Nahles said she was hopeful, but added: "Naturally I'm a bit nervous."
Volunteers began counting the postal ballots at 10 pm (2100 GMT) and worked through the night, with party leaders due to announce at 9 am whether Germany's longest post-election interregnum is over.
TRT World's Sarah Morice reports.
Germany, Europe's economic powerhouse, has faced months of uncertainty after voters returned a fragmented parliament. An earlier attempt to form a Merkel-led three-way coalition collapsed.
At the urging of President Frank-Walter Steinmeier, the SPD's then leader Martin Schulz promised members the final say on whether to repeat a "grand coalition" with Merkel's conservatives after first announcing he would lead the party into opposition.
Rejection of another tie-up with the conservatives "would be a disaster for Germany, the SPD and, above all, for Europe," SPD lawmaker Thomas Oppermann told Die Welt newspaper, forecasting that 55 percent of members would back a coalition.
If members vote "Yes", Merkel could be sworn in as Chancellor by mid-March.
Merkel has seen several junior coalition partners off over 12 years in office and many fear the SPD could dwindle into irrelevance if it props up her government for another four years.
SPD youth wing
The SPD's Jusos youth wing has been lobbying party members to vote against a re-run of the grand coalition, arguing they would do better to rebuild in opposition after the party's drubbing in the September election.
A "No" vote would throw the ball back into Steinmeier's court, who could push the pro-business Free Democrats (FDP), whose leader Christian Lindner torpedoed the earlier three-way talks, to have another go.
But Lindner told the Funke newspaper group, a new election would be the best solution if SPD members voted 'No'.
On another option, a Merkel-led minority government, he said the FDP would examine legislative proposals on a case-by-case basis, but added: "Such a government would probably only last a few months anyway."
Manfred Weber, deputy leader of the Bavarian CSU, sister party to Merkel's Christian Democrats (CDU), also favoured a new election in the event of a "No", arguing a minority government would impair Germany's standing in Europe.
"Germany would remove itself as a serious factor in Europe and the world," he said.