Olaf Scholz, appointed SPD interim leader on Tuesday, said European Union reform plans included in the coalition agreement were a strong reason to back the deal.
"I depart this office without bitterness or resentment", he said, nearly a year after he was elected as SPD party head with 100 percent of the vote.
With many SPD rank and file harboring misgivings about sharing power with Merkel again, the result of the vote, due on March 4, is wide open. Schulz proposed the head of SPD's parliamentary fraction, Andrea Nahles, as his successor, but it's now up to the party's congress to decide on its leadership.
The embattled leader of the SPD suddenly gave up plans to become the next foreign minister on Friday, hoping to shore up support among SPD members for a new coalition with Mrs. Merkel's Christian Democrats.
The manner in which Schulz originally anointed Nahles as his successor - without broad consultation - rankled with many in the party and resulted in Scholz being appointed as interim leader on Tuesday.
Woman Claims She Flushed Hamster on Spirit Air's Advice
Why the sudden act of animal cruelty towards an animal she specifically brought to the airport for its emotional significance? She's purchased the lovable rat subsequent to building up an expansive however benevolent development in her neck a year ago.
But Oettinger also conceded that this would be her final term as chancellor, if the new "grand coalition" goes ahead, addressing a succession debate that is starting to take hold as the CDU starts to look ahead to a post-Merkel era.
Schulz is expected to take a seat on the SPD's backbenches.
Germany has been without a proper government since an inconclusive election last September, which saw Merkel's conservatives and the centre-left SPD both lose seats and a far-right party enter the Bundestag lower house for the first time.
In a cartoon on Tuesday, the Sueddeutsche Zeitung daily showed Nahles with a whip riding an SPD snail.
The SPD has slid into disarray since its leaders struck the coalition deal last week, blighted by bitter divisions over whether to join the coalition, a loss of confidence in outgoing leader Martin Schulz and discontent over the succession process. After fierce criticism from some former allies, Schulz ditched plans to take the post of foreign minister.
Among supporters of Merkel's Christian Democrats and their Bavarian sister party, the Christian Social Union (CSU) - with whom the SPD are poised to form a with coalition government - 71.6 percent were critical of the SPD's ability to govern.