SPD chairman Lars Klingbeil left the further political fate of Defense Minister Christine Lambrecht open on Sunday.
He does not comment on media reports, said Klingbeil on ZDF, referring to reports from several media that the SPD politician would announce her resignation as minister on Monday.
Lambrecht has "great support," said Klingbeil.
What the SPD has to decide, they will decide collectively and then announce.
Klingbeil was the first leading SPD politician to comment on the defense minister since the first reports of his resignation on Friday evening.
The SPD leader is himself considered a possible successor to Lambrecht.
There is also speculation about the appointment of the previous military commissioner Eva Högl, the parliamentary state secretary in the defense ministry, Siemtje Möller, and the previous labor minister Hubertus Heil.
ARD also named Chancellor Wolfgang Schmidt as a possible successor and reported that it was unclear whether the successor would be announced when the declaration of resignation was expected on Monday morning.
In its coalition with the Greens and the FDP, the SPD is at the top of the defense department.
On January 20, the western allies meet in the so-called Ramstein format to discuss further arms deliveries to Ukraine.
Chancellor Olaf Scholz will deal with other difficult questions, namely whether Germany will supply Leopard 2 tanks or whether other countries will be allowed to hand over Leopard tanks to Ukraine.