Failure to secure a coaliion government could spell the end of German Chancellor Angela Merkel's grip on power
Elections in September had left the veteran leader weakened and without a majority as some of her party's voters turned to the far-right AfD because of anger over her liberal refugee policy.
The disputed decision to let in more than a million asylum seekers since 2015 is also proving to be a stumbling block as she seeks an alliance with an unlikely group of parties spanning the left and right of the political spectrum.
Chancellor Angela Merkel will make a last push Sunday to forge a government, in a twin battle to save her political future and avert fresh elections that could destabilise Germany and Europe.
Merkel's conservative CDU party and its Bavarian allies the CSU, hoping to find common ground with the pro-business FDP and the Greens, have given themselves until Sunday to clinch a deal.
If they do not, Germany would have to hold new elections in 2018, as the centre-left Social Democratic Party has ruled out returning to a coalition with Merkel after suffering a humiliating loss at September's elections.
"Today is the last day of these exploratory talks. We must decide," CSU chief Horst Seehofer said as he entered into talks Sunday.
Greens leader Cem Ozdemir also said that "so far we've been in extra time, but today it's the penalty shoot-out."
"Europe is anything but out of the crisis. Now it's time to ask ourselves the question, Would France's President Macron have to take the coal out of the fire because Germany is missing, or would Germany be capable of taking action?" Ozdemir added.
Merkel, who has years of gruelling EU negotiations under her belt, now needs to see through what is likely the most important weekend of her political life.
"Today is not only about (the coalition), but also a day of destiny for Angela Merkel. If she fails to forge a coalition, then her chancellorship is in danger," the top-selling Bild newspaper said.
Frank Decker, a political scientist at the University of Bonn, also had no doubt about what is at stake.
"It is absolutely in her interest for this government to come into being, because failure would spell her end," he told the Phoenix news channel.
A poll by Welt online found that 61.4 percent of those surveyed said a collapse of talks would mean an end to Merkel as chancellor. Only 31.5 percent thought otherwise.
- 'Coalition of mistrust?' -
Merkel, in power for 12 years, had initially set a Thursday deadline to decide if the motley crew of parties had found enough common ground to begin formal coalition negotiations.
But the talks went into overtime without a breakthrough.
Key among sticking points is the hot-button issue of immigration.
The CSU, which lost significant ground in Bavaria to the AfD and faces a crucial state election next year, wants to limit the number of future arrivals at 200,000 a year.
German media reports said the Greens were ready give way on the CSU's demand, but in return, they insist that war refugees -- who are granted only temporary protection -- should be allowed to bring their family members to Germany.
"We will not accept that people who are already getting a lower status of protection by law are also excluded from family reunions. That is inhumane," Greens negotiator Juergen Trittin told the Bild am Sonntag newspaper.
The Greens will be wary of making concessions ahead of a party congress in a week's time, and rank-and-file members can still torpedo any deal that they deem unsatisfactory.
Germany's President Frank-Walter Steinmeier played down the conflict, telling the Welt am Sonntag newspaper that if negotiators are "battling hard over major questions like migration and climate change, that may not be a bad thing for democracy".
There is "no need to start holding panic debates about new elections," he said.
He added that "all sides are aware of their responsibilities. And this responsibility means not returning their mandate to voters."
If the potential tie-up, dubbed a "Jamaica coalition" because the parties' colours match those of the Jamaican flag, comes together, it would be the first of its kind at the national level.
But questions abound about how stable it would be.
SPD parliamentary chief Andrea Nahles told the Funke media group she believed such an alliance would be "a coalition of mistrust, in which there is constant conflict, where each one plays his own cards, and where there isn't teamwork."
Decker, the political scientist, said he "wouldn't place a bet on whether this government will hold together for four years."