At an extraordinary congress in the western city of Bonn, 600 delegates from the centre-left SPD and its 45-member board will have their say on entering into formal talks for a renewed alliance with Merkel's centre-right CDU/CSU bloc.
The stakes could scarcely be higher for Merkel as a "no" vote would leave her with the option of leading an unstable minority government -- a prospect she has baulked at -- or face snap polls.
Horst Seehofer, leader of Merkel's Bavarian CSU sister party, said a thumbs down from the congress would be "a political catastrophe" for Germany.
"I can only appeal to everyone to get it together and help bring about a government," he said in a recent interview with Bild newspaper.
The outcome is far from certain after leading SPD members savaged a coalition blueprint painstakingly thrashed out by the three parties this month, complaining it contained too many concessions on issues such as migration, taxes and healthcare.
With his career on the line, SPD leader Martin Schulz has been criss-crossing the country trying to sell sceptical party members on the idea of another "GroKo", as the grand coalition is known in German.
In a letter to party members Friday, Schulz stressed the vote's "enormous importance for the future of Germany... all of Europe and the SPD".
He told Der Spiegel news weekly that a defeat would "very quickly" trigger new elections in which voters would be likely to "punish" the conservatives but also the SPD.
Pitted against him is the fiery chief of the party's youth wing, Kevin Kuehnert, who has been on his own "#NoGroKo" tour, arguing that the party would be better served spending the next four years in the opposition.
"I'm very optimistic that we have a realistic chance at winning this vote," he told reporters on Thursday.
Germany has been stuck in political limbo since September's inconclusive general election saw mainstream parties bleed support to the far-right AfD, which tapped into anger over Merkel's open-door refugee policy.
Bruised by his party's worst result in decades, Schulz initially vowed to go into opposition but then caved to pressure to reconsider after Merkel's attempt at a novel tie-up with two smaller parties collapsed in November.
The U-turn angered many grassroots SPD'ers, who believed some time on the opposition benches would help the 150-year-old party regain its fighting spirit.
Merkel, often dubbed the world's most powerful woman, has been largely absent from the global stage as the domestic drama has played out, and she is increasingly described by commentators as entering the twilight of her rule.
With her hopes for a fourth term now squarely in the hands of the SPD, the veteran leader has urged party delegates to make "a responsible decision" and back the "fair" coalition roadmap.
The 28-page in-principle agreement that lays the basis for future government policies promises more spending on childcare, education and pensions as well as joining France in a push to reform the EU.
But critics within the SPD have bemoaned the party's failure to secure a tax hike for the rich or a restructuring of the country's two-tier health care system -- two major campaign pledges.
They have also voiced dismay at the document's mention of limiting asylum seeker arrivals to around 200,000 a year, a flagship CSU demand.
Schulz has since insisted that many details still need to be ironed out and proposed that any future coalition government be put under review after two years.
A Forsa survey published Thursday found that 61 percent of SPD voters were in favour of moving on to formal coalition talks. The same poll found that the SPD would score just 18 percent if elections were held now, below the 20.5 percent garnered in September.
Even if party delegates give the green light to open formal coalition talks, another hurdle looms on the path to a new government. Schulz has pledged that any final coalition agreement will still be put to a vote by more than 400,000 rank-and-file SPD members.
By AFP's Marion Payet and Michelle Fitzpatrick