September's inconclusive general election left the camps, which span the political spectrum, scrambling to form a stable government under Merkel, with the outcome of negotiations highly uncertain.
"We have four parties sitting at the table that are really very far apart," said Katrin Goering-Eckardt of the left-leaning Greens on the talks with Merkel's conservative bloc and the pro-business Free Democrats (FDP).
The goal is to cobble together by year's end an unprecedented federal "Jamaica coalition" -- nicknamed after the parties' colours, which match those of the Caribbean nation's flag.
Merkel, who emerged from the poll weakened, will size up progress of the exploratory talks on Friday, with the stakes extraordinarily high for all involved.
A failure of the negotiations could trigger new elections, which would likely cost Merkel's conservatives particularly dearly and further boost the far-right Alternative for Germany (AfD) party.
"If the alliance does not come together, Merkel is finished," weekly newspaper Die Zeit said.
The talks, which started slowly last month, are now gathering pace. But last week saw bitter clashes on hot-button issues including immigration and climate protection.
There was so little progress that the negotiators decided largely to leave those topics out of this week's discussions in the hope of making headway on less contentious points.
'Mother Teresa' type
The personal chemistry among the wildly different personalities in the mix has also proved volatile.
FDP negotiator Wolfgang Kubicki, who annoyed the Greens' Goering-Eckhardt at the start of the talks with a rakish kiss of the hand, accused her this week of being a self-righteous "Mother Teresa" type.
"You get the feeling that if you have a different opinion you are a bad person," he complained.
Alexander Dobrindt of the CSU, the Bavarian sister party to Merkel's Christian Democrats, accused the Greens of "provoking a breakdown" of the negotiations in sticking to a welcoming stance toward refugees.
"Anyone who fails to see the AfD's success and its links to the refugee crisis is not really serious about forming a government," he said, referring to the anti-immigration party whose support surged to nearly 13 percent in the election.
Political scientist Ursula Muench dismissed many of the broadsides as play-acting by seasoned professionals.
But she said there were nevertheless "fundamental" differences, particularly between the CSU and the Greens, on globalisation and cultural identity that would be extremely tough to bridge.
"These aren't about little compromises you can make -- they revolve around the question 'how open or sheltered do you want to make German society?'" Muench, who runs the Academy for Political Education near Munich, told AFP.
"Even if you manage to bend over backwards, how can you convince your own base that you still represent their interests?"
Conspicuously absent from the sniping has been Merkel, Forbes magazine's most powerful woman again this year, whose above-the-fray style has kept her at the helm of Europe's top economy for 12 years.
'Decisive rounds of poker'
Participants in the talks say that despite the flame-throwing in the media, the atmosphere set by Merkel behind closed doors is "business-like" and pragmatic.
"This is not our desired coalition. But if we spend five weeks in talks and then say 'never mind', we'd better have a good reason to give the voters," one negotiator said, speaking on condition of anonymity.
He put the odds of success at 80-20 in favour.
The camps are racing toward a November 25 deadline, when the Greens' notoriously combative rank-and-file will vote on whether enough progress has been made to enter into full-blown coalition talks.
The negotiators can point to a handful of accords thus far on issues including maintaining a balanced budget, working toward full employment and fighting poverty in old age.
The hope is that the risk of no deal will focus minds to tackle the thorniest items on the docket.
"The parties are warming up for the decisive rounds of poker," the daily Magdeburger Volksstimme said.
"It was was clear from the start that the road to Jamaica would be tough and fraught with risk. Don't expect miracles."
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