"The biggest risk would be that we repeat what we did the last time", Opperman said, warning that a passive SPD stance could result in even worse election results after four years.
Chancellor Angela Merkel vowed to work with the Social Democrats for the "good of Germany" as the centre-left party agreed to join her new coalition, but the veteran leader will go into her fourth term with weaker cards than before. Party leaders say they expect a "yes" vote to the coalition deal they hammered out with Merkel and her allies.
"Merkel needs to appease a weakened and insecure SPD without alienating her own CDU and the CSU too much", said Carsten Nickel, managing director at consultancy Teneo Intelligence.
In power since 2005, she has led Germany and the European Union through the financial and debt crises, but her waning authority at home could complicate efforts to deepen integration in the euro zone.
Sixty-six percent of the estimated 463,000 SPD party members who participated in the coalition vote opted to continue with the partnership; 34 percent voted against it.
To stop the SPD from leaving the coalition, Merkel must deliver on those points in the coalition deal that are most dear to the Social Democrats: healthcare reform and investment in education to meet the challenges of the digital age.
But her troubles are far from over.
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"This was a really important democratic decision for our country", said acting Social Democrat leader Olaf Scholz, who is in line to become Germany's next finance minister and Merkel's deputy.
European partners waiting impatiently for Germany to end its longest stretch of coalition haggling since the end of the war heaved a sigh of relief, with French President Emmanuel Macron calling the SPD decision 'good news for Europe'.
German Chancellor Angela Merkel casts her ballot at a polling station in Berlin, Germany, during the federal elections, on September 24, 2017. The CDU/CSU and SPD have governed in a grand coalition since 2013.
With Merkel's bloc and the second-place Social Democrats in government, the right-wing Alternative for Germany, or AfD, now represents the biggest opposition party in Parliament, giving it a prominent platform to attack the chancellor.
The joyless partnership was evident in the sombre faces of leading SPD members Sunday when they announced that their card-carrying members had signed off on their plan to again join Merkel in government.
Merkel also appreciates the importance of a future government for Europe. He has also advocated a sharp conservative shift in a bid to woo back voters from the AfD, which garnered almost 13 per cent in the September election.
For now, Merkel is hanging on and is due to be re-elected as chancellor by parliament on March 14 as President Frank-Walter Steinmeier, according to procedure, on Monday nominated her for the post.