SPD delegates voted by 362 "for" to 279 "against" to move ahead with negotiations after the centre-left party's leaders agreed a preliminary coalition blueprint with Merkel's conservative bloc earlier this month.
"You can't find anyone in the party who's excited about another grand coalition", said Kühnert, leader of an angry cabal of SPD left-wingers and many younger members.
Put simply, last September's general election was a disaster for Germany's Social Democratic Party (SPD).
Germany and France want to boost economic, social, political and technological cooperation by upgrading their bilateral agreement, known as the Elysee Treaty, concluded 55 years ago. Nahles' voice trembled as she recalled this, saying that basic pensions are a big political achievement.
"The Union now knows very well that (after) an extremely narrow result, this still has to go through an SPD membership ballot, so the SPD signature on this coalition agreement must be very clear", Roemmele told German public television station Phoenix.
Merkel's decision in 2015 to allow in large numbers of asylum-seekers caused major political friction and boosted the nationalist Alternative for Germany party, which previous year became the third-largest in parliament after campaigning on a shrill anti-migration and anti-Merkel message.
The SPD suffered its worst result in decades but remained the second-largest party in parliament.
Many voters were unhappy with the decision to continue coalition talks, noting SPD leader Martin Schulz failed to produce any truly Social Democratic policies in the first round of negotiations. But where is the Schulz who represents the sovereign agency of the SPD as a political party? It's also helps to explain why party officials across the political spectrum view the third option with a sense of trepidation.More news: Donald Trump's appointee resigns after saying: 'I don't like gay people'
"If we can achieve something good for people in this country, if we can achieve something good for the peoples of Europe, then we should do it", SPD Chairman Martin Schulz said in his closing appeal before the vote.
On Thursday, at its last meeting before Sunday's crucial vote, the Berlin wing of Jusos was marvelling at the success of their campaign and discussing tactics over soft drinks.
"A "No" wouldn't lead to chaos, it would lead to democracy", she shot back in a brief speech, as many in the crowd cheered.
Merkel herself still has another hurdle to overcome before she can be sworn in again as chancellor: The SPD's about 450,000 grassroots members still have to vote on any coalition deal that their party and the CDU-CSU agree to in the upcoming negotiations.
Even if the SPD does vote in favour of starting coalition talks, a new government is far from guaranteed.
Of course, there is another possibility for Merkel and Seehofer besides forming a coalition with the European federalist and socialist Schulz: the CDU could form a minority government with, for instance, the classically liberal (conservative) FDP.
Even if efforts to set up a formal coalition pact fail and lawmakers elect Merkel chancellor of a minority government, she could broadly count on SPD and Green party support for her European agenda. This is precisely why some SPD members are concerned about going into a new grand coalition.
"I'm asking for your trust", Schulz told the delegates, adding that turning away from further coalition talks would be "reckless".