The referendum for the 460,000 members of the crisis-hit SPD is the last hurdle for veteran leader Merkel as she seeks to launch a new government, five months after an inconclusive election.
If the rank-and-file of the 153-year-old labour party give the thumbs up — with the results to be announced on March 4th — Merkel is set to launch her fourth-term government within a few weeks.
If they vote no in the postal and online ballot, Europe's biggest economy faces more political limbo and likely snap elections that would threaten a speedy end to Merkel's 12-year reign.
Despite weeks of turmoil and bitter infighting, the SPD leadership is hopeful its members will back the proposal for a continuation of the current right-left “grand coalition” government, known in German as the “GroKo”.
But the outcome is far from certain, given the volatile mood in the party which scored its worst post-war result of just 20.5 percent in the September 24th election.
- Far-right AfD now the second most popular party in Germany, poll shows
- Here's how a woman could be about to lead the SPD for the first time
The party's youth and left wings are driving a passionate #NoGroKo campaign, arguing that the party must rethink what it stands for and rebuild as a combative opposition force.
The SPD's credibility has been badly bruised by a series of U-turns, which last week saw election loser Martin Schulz resign as leader after less than a year in the post.
The party's ratings are in freefall, with latest polls giving it just 15.5 percent support — narrowly behind the anti-immigration Alternative for Germany party.
Merkel's conservative CDU/CSU bloc remained the strongest political force with 32 percent support.
The AfD cheered having polled as Germany's second strongest party, and its parliamentary leader Bernd Baumann vowed the populists were now training their sights on “our next target, the CDU”.
'End of big parties'
The mass-circulation Bild daily called the Insa survey it commissioned “a bitter blow” for the SPD, one of Germany's two traditional mainstream parties.
The tabloid-style paper also ran a mocking front-page report on how the SPD had been tricked into approving the membership application of a dog and had mailed brochures and invitations to its owner's address.
An SPD spokesman said the party was looking to cancel the “fake membership”.
Amid the political chaos, Germany — long seen as a bulwark against populism, in part due to its Nazi history — is now experiencing the tectonic shift of mainstream parties in decline long observed in other western democracies.
The ballot-box pain of Germany's two biggest parties was in large part a result of the rise of the anti-Islam AfD which railed against a mass influx of refugees that peaked in 2015 under the previous grand coalition.
The populists won almost 13 percent of the vote with their angry demand that “Merkel must go” and their protests against the two establishment parties which, they argue, have effectively merged into a GroKo mega-party.
“Something is coming to an end in Germany: the age of the traditional big parties,” news weekly Der Spiegel said in its latest editorial, stressing that Merkel's CDU faces the same change, albeit less dramatically.
The shift, Spiegel judged, is in line with “the fragmentation of society, greater individualism, the dissolution of traditional socio-economic groups… and the logic of social media”.
“The trend has reached Germany relatively late,” the magazine added, pointing to the Netherlands, Belgium and France, where President Emmanuel Macron has “swept away the old party system”.