Germany's oldest political party is also the one stuck with the biggest existential crisis. Some 600 Social Democratic Party (SPD) delegates — from the various state party organizations — are due in Bonn on Sunday to vote on whether to move into formal coalition negotiations with Angela Merkel's Christian Democratic Union (CDU) and its Bavarian sister party, the Christian Social Union (CSU).

"It's not an exaggeration: The world is looking at Bonn this Sunday," former SPD leader and current Foreign Minister Sigmar Gabriel told the Bild newspaper on Wednesday. Meanwhile his successor as party leader Martin Schulz, helpfully reinforcing the sense of political melodrama, added that "an epoch change in European politics is within reach."

The chancellor herself may well be clinging to her TV set in her modest apartment back in Berlin on Sunday, too. Although the delegates' decision is not final (the SPD's 440,000 members will also vote), what happens in Bonn, itself gripped by an actual storm this week, will probably show which way the wind is blowing.

Should the SPD say "no" to the "GroKo" (as the CDU/CSU grand coalition with the SPD is nicknamed in Germany), Merkel will be left with two options. Neither of them are entirely to her taste: a CDU/CSU-led minority government — which will see the conservatives fishing around for a parliamentary majority among various parties from one draft law to the next — or a new election, which may prove just as inconclusive as the one last September, and which may herald the end of Merkel's chancellorship.

Party in crisis

There is no less at stake for the SPD. The venerable center-left party's ongoing identity crisis (which, rather un-crisis-like, has been running for several years) has now assumed full-on calamity status: This week, the SPD dropped to 18.5 percent in one opinion poll — 2 percentage points below last September's worst-ever election result (though, in fairness, most of the other polls still had the Social Democrats clinging on to the 20-percent mark).

As a result, the SPD spent most of the week denying the very obvious rift in the party. Schulz, whose one-year tenure as party leader would surely be up in the event of a "no" vote on Sunday, has already admitted that he couldn't be sure that he had enough delegates on his side. In a TV talk-show this week, SPD parliamentary leader Andrea Nahles estimated that a third of delegates are still undecided. Some, like Berlin's SPD, have already declared that they will vote "no."

Nahles and other party heavyweights, including Gabriel, are firmly backing the 28-page policy paper that came out of preliminary talks with the CDU/CSU last week, and which is supposed to be the blueprint for the coalition negotiations.

Issues at stake

Leading the line for the "NoGroKo" campaign is the SPD's "Young Socialists" (Jusos) leader Kevin Kühnert, the 28-year-old politics student who can be counted on to deliver an impassioned speech on Sunday.

He, like many grass-roots SPD members, believes that another four years tethered to the back of Merkel's centrist ship would mean virtual death to his party, and he was none too pleased with the many concessions that he says Schulz made in his negotiations with Merkel.

Kühnert's noisy opposition to GroKo has made him a popular media figure this week. "We can't keep making the same mistakes," he told a political talk show on broadcaster ZDF Thursday night. For him, as for many SPD members, it was the long-term alliance with the conservatives that had led the party into this mess in the first place. 

The initial coalition negotiations did indeed feel like the left-wing of the SPD had been a little left out. The SPD's demand that the top rate of income tax would be raised, rather modestly, from 42 to 45 percent, for instance, found no traction with the conservative negotiators. The same went for the Social Democrats' plans for a "citizens' insurance scheme," which would have guaranteed everyone in Germany certain basic health care standards regardless of whether they are publicly or privately insured.

Equally, the right-wing in the CDU/CSU corner won their much-contested asylum-seeker cap (at between 180,000 and 220,000 a year, which is above the current rate anyway).

A new SPD?

And yet, as Schulz pointed out in a long Facebook post on Friday morning, the SPD also got a few of their own points through: health insurance contributions would once again be equally divided between employer and employee, the federal government would be able to finance school renovations, and a new program in affordable social housing would be launched.

Indeed, some analyses did find that the SPD had got some favorable compromises out of the initial talks. News agency DPA found the SPD's "handwriting" on pension reform plans, which would see a basic state-guaranteed pension plan for low-income earners — even if it doesn't include the long-term guarantee the party was hoping for.

Similarly, the social infrastructure investments included in the coalition paper also spoke of significant SPD influence: €2 billion ($2.4 billion) for new social housing, another €2 billion for new all-day schools, and €3.5 billion for kindergartens. 

But as with its successes during the last coalition government (finally bringing in a minimum wage, for instance), the SPD leaders are once again struggling to sell these modest gains as real results. Without a single high-profile policy victory, as the conservatives scored with the immigration cap, Schulz will do well to bend skeptical delegates his way.

Moreover, credibility is a problem for the SPD leader. On election night, the roundly defeated Schulz announced that his party would definitely go into opposition this time round, and the universal opinion — for SPD members and pundits alike — was that this was the only real option. There was a sense that the SPD had been sucked dry of ideas and identity by Merkel's effortless centrism, and it needed to be out of government to find itself again.

But the failure of the Jamaica coalition negotiations in November left the SPD with a difficult decision to make — and led Schulz to roll the dice one more time with talk about "stability" and "responsibility."

And yet, with the young socialists warning of the fall of the party, Sunday's vote is on a knife-edge.