Germany’s ruling parties are reeling from their second electoral upset in a fortnight, after voters in the key state of Hesse appeared to have abandoned them in droves, plunging both parties of Angela Merkel’s coalition government into renewed crisis.

Early results on Sunday from a regional election in Hesse seen as decisive for the future of Germany’s wobbly coalition government showed Dr Merkel’s Christian Democrats (CDU) slumping to 28 per cent. It would be the party’s worst showing in the state since 1966 and a drop of 10 per cent since Hesse last went to the polls in 2013.

Exit polls showed Dr Merkel’s coalition partner in Berlin, the Social Democrats (SPD), tanking to 20 per cent of the vote, coming in just half a percentage point ahead of a resurgent Green party. The result, the SPD’s worst ever in the state, will pile pressure on its party leader, Andrea Nahles.

German Social Democratic Party leader Andrea Nahles as first exit polls from the Hesse state election come through in Berlin. Photograph: Hannibal Hanschke/Reuters

Ms Nahles said the party was planning a new roadmap for their national government with Dr Merkel. The SPD will then use the roadmap to measure the progress of the coalition at a mid-term review next year – half-way through the parliamentary term.

“We could then gauge the implementation of this roadmap at the agreed mid-term review, when we would be able to clearly see if this government is the right place for us.”

The trouncing for the coalition parties follows a disastrous result in Bavaria widely seen as a protest against the failings of the Berlin government. As in Bavaria, the big winner appeared to be the environmental, pro-immigration Green party, which doubled its voter share in the state to 19.5 per cent at the expense of both coalition parties. The far-right Alternative fur Deutschland (AfD) also made gains, comfortably entering Hesse’s parliament for the first time on 12 per cent.

If it stands, the result would see the state’s CDU-Green coalition scrape a majority, putting an end to speculation over the future of the CDU state premier and close Merkel ally, Volker Bouffier. With tensions running high in the CDU, some members had implied that if Mr Bouffier falls, Dr Merkel may struggle when she stands for re-election as party leader at its conference in December.

“We are in pain because of the losses but we also learned that it is worth it to fight,” a Mr Bouffier told supporters.

“The message to the parties ruling in Berlin is: People want fewer disputes and more focus on the important issues,” he said.

Hesse, the seat of Germany’s financial centre Frankfurt, is a swing state traditionally seen as a bellwether for national politics. For the past 20 years it has been ruled by CDU-led coalitions. At Hesse’s previous regional election in 2013, the CDU secured 38.3 per cent of the vote and the SPD 30.7 per cent.

On the campaign trail this week Dr Merkel was at pains to play down the significance of the regional vote for her party, government and chancellorship, protesting that “not every regional election can be stylised into a little national election” – Guardian