ON SUNDAY Germany’s Social Democrats (SPD) gather in Bonn, by the River Rhine, to decide whether to proceed to formal negotiations with Angela Merkel’s Christian Democrats (CDU) and their Christian Social Union (CSU) allies. The choice will be made around mid-afternoon by 600 delegates comprising groups representing each of the 16 federal states according to population (the largest, North Rhine-Westphalia, sends 144).

It happens that the venue of the conference, in a southern suburb of the old West German capital, is within walking distance of Bad Godesberg. It was here, in 1959, that the SPD abandoned its old Marxist theories and embraced reform capitalism. This turning point paved the way to the election of Willy Brandt (pictured, above, addressing the conference in his capacity as West Berlin’s mayor) as the federal republic’s first SPD chancellor a decade later.

To listen to some in the party, Sunday’s meeting could be almost as fateful; another turning point in the party’s history at which its identity and purpose hang in the balance. It is no exaggeration to say that opponents of further talks with Mrs Merkel, led by the Young Socialists, the party’s youth wing, consider another round as her junior partners a threat to the SPD’s very survival as a prospective lead party of government.

As I describe in this week’s issue of The Economist, the battle between supporters and opponents of a new “grand coalition” has been hard-fought, and at times emotional. SPD insiders privately expect that a majority, perhaps around 60%, of delegates will back the leadership and endorse the talks. But what are the main arguments on both sides?

The case against coalition talks

Germany's election on September 24th was a clear rejection of “more of the same”. The collective vote-share of the grand coalition parties fell from 67% in 2013 to 53%. Voters are fed up of the soggy consensus at the centre of German politics, which explains the rise in support for all the smaller parties with clearer identities: the Free Democrats (FDP) and Alternative for Germany (AfD) on the right, the Left and the Greens on the left.

Grand coalitions are meant to be the exception in Germany but have governed the country for eight of the past twelve years. Another such spell would fuel political disillusionment and with it the far-right, anti-immigrant AfD, which would become the main opposition party in the Bundestag and thus reap goodies like the chairmanship of the powerful Budget Committee.

And what would a new grand coalition achieve? The preliminary blueprint agreed by the parties last week is broadly unambitious and omits major SPD demands like higher taxes on the rich and a consolidated public health insurance system. Though mostly drafted by SPD figures like Martin Schulz, a former European Parliament president and now the party's leader, the section on Europe is long on rhetoric and short on detail. Crucial euro-zone reforms, like the completion of banking union, go ominously unmentioned.

An SPD “no” to talks with Mrs Merkel could, it is true, lead to a minority government, which is alien to the German system, or new elections that could cost the party further seats. But neither prospect should inspire terror.

Minority government is not unusual in Scandinavian countries, like Denmark, with similar political systems to that of Germany. Under such a CDU/CSU administration the SPD could support the things it likes, like increased public investment and a Franco-German reform push in Europe, from opposition. Mrs Merkel says she would prefer new elections if the talks fail, but many in Berlin suspect that she would in fact opt for a pure CDU/CSU government, bring new talents into her cabinet, pursue a limited list of policy goals (like a deal on euro-zone reform) before stepping down ahead of new elections, perhaps in 2019 or 2020. That could work well for the SPD, and refresh German democracy.

As for new elections this year, the SPD’s current polling values are a dismal couple of points below the party’s 20.5% result at the election, but may reflect the party’s ongoing flirt with a new grand coalition. Saying no to Mrs Merkel would give it new definition and possibly a corresponding surge of support—like the one it enjoyed in early 2017, in the first weeks of Mr Schulz’s leadership. In any case, even 18% at an early election this autumn would be better than falling to 15% or below at another post-grand coalition election in 2021—and letting the AfD into second place.

Opposition is an opportunity for the SPD: to renew, promote a new generation of leaders and reassert its left-of-centre identity ready for the lively clash of ideas with a post-Merkel CDU/CSU that the country so badly needs. This, a real choice between left and right, is the best long-term answer to the right-wing populists and the best basis for an internationally confident and effective Germany. As Brandt himself put it, it is time to “take a chance on more democracy”.

The case for coalition talks

A grand coalition would have a majority in the Bundestag. It has every right to govern. More than that, it has a responsibility to do so. Neither Europe nor the wider world will go on "pause" while the SPD sorts itself out (which it singularly failed to do in its last spell in opposition, from 2009 to 2013).

New elections would waste valuable months and might not change the coalition arithmetic, anyway. And minority government? Germany is not Denmark. It is the continent’s biggest economy. It needs a stable majority government and a chancellor capable of walking into international negotiating rooms without having to run a simultaneous negotiation with opposition MPs back in Berlin from her mobile phone. Moreover, the new Bundestag has a right-of-centre majority, so every time the SPD withholds its support it will be forcing the CDU/CSU to pass legislation with the backing of one or both of the FDP and AfD. In practice it may end up with some of the responsibilities of government, but none of the influence.

On Europe, in particular, timing is everything. It is hardly possible to imagine a French president more sympathetic to German values and interests than Emmanuel Macron. His speech urging a new push for European integration delivered days after the German election was an outstretched hand to Berlin. On its own the CDU/CSU, parts of which are paranoid about anything resembling a “transfer union”, cannot be trusted to take that hand—still less with the increasingly eurosceptic FDP at its side. To decline it would weaken Mr Macron and boost French populists of right and left. Don’t forget: France came within a few percentage points of a far-right versus far-left presidential run-off last year.

It may not feel like it in booming Germany, but the next crisis of the common currency is only a matter of time. Many of the causes of the last one—the “death loop” of banks and sovereigns, for example—remain unresolved. With Mr Macron still enjoying the afterglow of his political honeymoon in Paris, the euro-zone economy doing well and over a year to run until the next European Parliament election, there is an ideal window of a few months in which to start fixing the euro-roof in sunny weather. Such a window may not reappear before the next storm comes.

Yes, the provisional coalition paper was mediocre. But some tweaks and additions could be incorporated in formal talks. And it is not wholly anodyne: the guarantee of all-day care for all children of primary school age, for example, would be a really positive step in a country with below-average female employment. It also commits the parties to a more lively debating culture and envisages a review two years in. That might be a better time for early elections. On Europe the paper is an improvement on Mrs Merkel’s provisional agreements on the subject with the FDP and Greens in the autumn. It suggests a euro-zone budget, for example, and expanded crisis firewalls. Particularly if it wins the finance ministry in coalition talks, the SPD can guarantee and build on this agenda, perhaps along the lines of the package of proposals published earlier this week by a group of French and German economists. Brandt, the author of Ostpolitik, would surely see this moment for what it is: a test of his party’s commitment to the European ideal.

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Which side—which Brandt—is right? It depends on your priorities. If your greatest concern is the depoliticisation of German civic life and the effects of the AfD’s rise, you might well oppose a new grand coalition. If you think a prompt Franco-German deal on Europe’s future is more important, your inclination will be towards a “yes” vote. To govern is to choose. So too, in its way, is not to govern.