Every week for 30 years, sausage seller Alexandra Müller has set up her stall in the shadow of an ornate baroque church in the central German city of Fulda. She was here on a bright September day five years ago, when thousands thronged the nearby university square hoping to glimpse the then wildly popular Angela Merkel. And she was here last week, when the chancellor appeared in Fulda again, this time speaking behind closed doors to a few hundred diehard supporters.

“Merkel will helicopter in, talk to the big businessmen for an hour, and then leave,” said Müller, 50. “All the while I’ll be standing here, trying to sell my sausages. Merkel should come to the market. I’m looking for someone that’s nearer to the people. But what do I know? I’m just a sausage seller.”

Müller might swear she’s not important, but on Sunday, she and fellow voters in the central state of Hesse have the power to deliver a second electoral upset within a fortnight to Germany’s embattled ruling parties, potentially plunging both into fresh crises. The regional election is seen as decisive for the future of Merkel’s rickety coalition government.

Last-minute polling showed support plummeting for both her Christian Democrat Union (CDU) and coalition partner the Social Democrats (SPD) in a swing state traditionally seen as a bellwether for national politics.

Both parties were predicted to drop 10 points each since the state’s last regional election in 2013. Such a trouncing would come on the heels of a disastrous result in Bavaria that was widely seen as a protest against the failings of the Berlin government.

Facebook Twitter Pinterest Green candidate Tarek Al-Wazir at an election rally in Frankfurt. Photograph: Thomas Lohnes/Getty Images

“None of the parties are there for us,” said Müller, who has voted for both CDU and SPD in the past, but was still undecided. “What should I do? I have to vote, it’s my duty to stop the far right getting into power. But I also know I won’t be heard. I can vote for whoever I like; the politicians will still do whatever they want.”

Hesse, home to Germany’s financial centre, Frankfurt, has been governed by CDU-led coalitions for the past two decades. But polls have the party nosediving to 28%, a result that would end the state’s CDU-Green coalition and leave a question mark over the future of CDU state premier and close Merkel ally Volker Bouffier.

With tensions running high in the CDU, mutinous members have implied that if Bouffier falls, it may cost the chancellor vital votes when she stands for re-election as party leader at its conference in early December. But Merkel, who joined Bouffier on the campaign trail last week, was at pains to play down the significance of the regional vote for her party, government and chancellorship.

“Hesse, and what happens here, is being watched and considered from far beyond Germany’s borders,” Merkel told supporters on Thursday in Fulda. “I want to point out once again that on Sunday the vote is about Hesse. Afterwards we’ll talk again about Berlin.”

But that message doesn’t seem to have trickled down to voters in Fulda, where the CDU secured just under 50% in 2013. Some say they relish the chance to show their displeasure with Merkel’s coalition, which has limped from crisis to crisis since assuming power in March.

“For me this will be a protest vote against all this nonsense in Berlin,” said Heiko Becker, 48, a lifelong swing voter now leaning towards the anti-immigrant Alternative für Deutschland (AfD). “Things are good for us in Hesse, but no one cares about that. It’s all about going against Merkel nowadays. Lots of people are switching their votes.”

As in Bavaria, Hesse’s booming economy and low unemployment figures have not stopped voters ebbing away to the far right. Latest polls put the AfD on 12%, behind the top three parties, but enough to easily enter the state parliament for the first time. Yet as in Bavaria, it is the Green party that is set to make the biggest gains. Polls suggest the party has doubled its support in Hesse over five years, putting it neck and neck in joint second place with the ailing SPD on 20%.

“We’re definitely profiting from the weakness of the grand coalition,” said Marion Neumeister, head of Fulda’s Green party office, adding she had noticed a marked increase in voters actively approaching her asking to be persuaded to vote Green. “We hardly have to say anything. Look at what they’re doing in Berlin, they’re driving people towards us.”

The Greens are now polling at their highest nationwide since Japan’s Fukushima disaster in 2011. In the past, said Neumeister, local party members joked darkly that they’d only see such highs again if another nuclear reactor melted down. “Now we have our meltdown,” said Neumeister, eyeing the latest polls. “The CDU meltdown.”

But disillusion with stagnant Berlin politics isn’t only a problem for the CDU. For Hesse’s SPD, the coalition’s record is proving just as awkward.

“It seems a large part of the population doesn’t want this grand coalition any more,” said Sebastian Busch, who is standing for the SPD in Hesse’s western district of Rheingau. “In the long term, we have to bring people a vision of a future in which one can trust politicians to do what they say they will.”

The reckoning could come sooner rather than later. If the SPD is overtaken by the Greens in Hesse, party leader Andrea Nahles could be forced to step down. In the most extreme case, such a result could shock the SPD into leaving Merkel’s coalition, which would almost certainly trigger new elections.

“Coming in under 20% or behind the Greens would be two very strong symbols for the SPD, which is still reeling from its worst-ever result in Bavaria,” said Thorsten Faas, professor of political science at Berlin’s Free University. “The opponents of the grand coalition would use this to say we have to get out. There’s just no conviction, no enthusiasm for this project in large areas of the SPD.”

Facebook Twitter Pinterest Merkel with local CDU leader Volker Bouffier at an election rally last week. Photograph: Thomas Lohnes/Getty Images

But however miserable it is in the current government, the SPD’s dismal nationwide polling figures means the party has little to gain from voluntarily forcing an election.

“That would be the worst outcome for the SPD,” said Andrea Römmele, a communications professor at Berlin’s Hertie School of Governance. Merkel, she added, was likely to remain safe if the CDU could keep power in Hesse under a three-way coalition with the liberal Free Democratic Party and the Greens.

“The only way Angela Merkel won’t come out OK is if Bouffier doesn’t remain state premier in Hesse,” said Römmele. “The way the polls are now, that’s not likely. We’ll have a government soon in Bavaria and a government in Hesse, and that will be the end of the conversation for now.”