After German reunification in 1990, the Social Democratic Party (SPD) boasted nearly a million members. These days, that figure is just over 440,000. But the members who remain are set to play a crucial role in the future of the center-left party.

The Christian Democrats (CDU), the Christian Social Union (CSU) and the SPD have set a Sunday deadline for concluding coalition talks, though party leaders have agreed to a two-day grace period in the event they have yet to overcome key differences.

Read more: Merkel says 'serious differences' remain in coalition talks

Without the consent of SPD members, Germany will not be governed by a grand coalition. Once talks have concluded, a copy of the coalition treaty will be sent to each of them for review. When they go over the document, the key issue will be: Does the proposed government agenda bear the stamp of the Social Democrats in a clearly perceptible manner, or were the SPD's negotiators shortchanged by their conservative counterparts? 

As was the case four years ago, it's the SPD that has the last word when it comes to signing a coalition treaty with Chancellor Angela Merkel's CDU and its Bavarian sister party, the CSU. Members vote by letter, submitting a simple "yes" or "no." New members are allowed to vote as well, provided they join the party before February 6.

In fact, following the party's conference in January — which greenlighted formal coalition talks with a narrow majority — there has been an increase in applications for SPD membership. Whether the majority of those new members are for or against a renewal of the grand coalition, however, is difficult to gauge for the party's leadership.

Risky vote

In contrast to negotiations to form Germany's previous government in 2013, which 76 percent of the SPD's members approved, dissenting voices in the party have the opportunity to derail the current grand coalition at an early stage. Many Social Democrats already feel that some of their key demands are being overlooked, for example the issue of refugee family reunifications.

Read more: Major sticking points in Germany's coalition talks

In addition, they are at odds with party leader Martin Schulz's change of heart following the collapse of exploratory talks held by the CDU/CSU, the Greens and the laissez-faire Free Democrats.

In the immediate aftermath of the national election in September, Schulz had insisted that the SPD would enter the opposition in parliament. Now he wants to lead the party into another grand coalition, in which the SPD will serve as a junior partner to the CDU/CSU for the third time running — despite the painful experiences of the past.

Twice in a government under Merkel's leadership, the SPD has suffered at the voting booth in subsequent elections. According to current opinion polls, support for the SPD and Schulz have hit a record low.

Read more: Germany split on prospect of Merkel's fourth term as chancellor

What if a third grand coalition fails?

Should SPD members ultimately vote against forming another grand coalition, the ball will be in President Frank-Walter Steinmeier's court. The head of state, normally a mostly ceremonial role, will propose a chancellor candidate for election by the Bundestag — likely to be Merkel, whose party garnered the most votes in September's election.

The candidate needs an absolute majority to be approved. Should that threshold not be reached, another vote is held two weeks later. If an absolute majority is still lacking, parliament votes a third time, and then only a relative majority is needed. After that, Steinmeier would have to decide whether to appoint the elected candidate as the new chancellor of a minority government or dissolve parliament and call for a new election to be held within 60 days.

Thus far, the SPD leadership has avoided discussing the possibility of a new election, which could see the party fare even worse than the far-right Alternative for Germany. Instead, SPD leaders are campaigning for support for the coalition treaty, keeping under wraps, for the time being, which SPD politicians would receive a minister post in the new cabinet.

The beleaguered Martin Schulz, who initially wanted to avoid serving in a Merkel-led cabinet at all costs, is reportedly seeking the job of foreign minister — yet another about-face that could fan the flames of grand coalition opponents within the SPD.