Junior coalition partners to Merkel's Christian Democrats (CDU), the SPD elect their new leaders against a backdrop of disastrous poll ratings and internal division over their role in government.
The 425,630 party members have from Tuesday until November 29th to cast their vote, as they elect the first male-female leadership duo in the party's 129-year history.
In a leadership election dominated by one major issue, SPD members must choose between two pairs with differing views on the coalition government.
In place since 2017, it is the third so-called “grand coalition” since 2005, and remains unpopular among voters and SPD members alike.
Incumbent finance minister Olaf Scholz and his partner Klara Geywitz have argued for a continuation of the coalition until the next elections in 2021, though Scholz has also said it should be the last of its kind.
A former mayor of Hamburg and currently Merkel's vice-chancellor, Scholz is considered a potential candidate to run for Chancellor himself if he and Geywitz win.
Challengers Norbert Walter-Borjans and Saskia Esken, meanwhile, have earned support from the party's left and youth wings with a more critical stance on the coalition.
In a televised debate on Monday, Esken said that she would be prepared to let the government fall if the CDU and their Bavarian sister party refused to renegotiate the coalition agreement.
The poll marks the climax of a five-month process, which began when former party leader Andrea Nahles stepped down in June.
Nahles' resignation came after the once proud SPD picked up just 15.3 percent of the vote in May's EU elections, confirming their steady slide in poll ratings in recent years.
Latest polls put the SPD around five points behind the Green Party, which has jumped to 20 percent under its own male-female leadership duo this year.
Results of the vote will be published on November 30th, with the new leaders to be formally confirmed at the party congress on December 6-8th.
During the congress, members will also have the chance to vote on the future of the coalition.