BERLIN (Reuters) - Germany’s Social Democrats (SPD) signaled on Wednesday they could quit their coalition with Angela Merkel’s conservatives if she is forced out as chancellor, piling pressure on their partners to avoid a snap election as they pick a new leader.

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After Merkel’s protegee Annegret Kramp-Karrenbauer gave up her ambitions for the top job on Monday, the Christian Democrats (CDU) are embarking on choosing a new leader and chancellor candidate for the next federal election due by Oct. 2021.

The possibility of having a rival as party leader while she remains chancellor may be unworkable and force Merkel, who will not seek re-election after leading Europe’s biggest economy for around 15 years, to stand down early.

This could trigger an early election, not least because the SPD have made clear their coalition deal is only with Merkel.

SPD General-Secretary Lars Klingbeil said the party entered the coalition with Merkel. “And we will leave the coalition with her - as planned at the next regular federal election,” he said in some of the clearest comments yet from a senior SPD figure.

“I am aware of no other election date,” he added.

The fragile coalition has already come close to collapse several times and the selection last year of two leftists as possible new SPD leaders has left the alliance even more shaky.

Many lawmakers want to avoid the upheaval of an election during Germany’s tenure of the rotating presidency of the EU in the second half of this year.

Klingbeil said he expected the conservatives were aware of their responsibilities and was “not running away from the EU presidency”.

The SPD may find it impossible to work with at least two of the potential conservative candidates, Friedrich Merz and Jens Spahn, who are further to the right of the CDU than Merkel, although others may be more palatable.

While the SPD wants to stay in government with the conservatives for the full legislative term, it is ready to fight an election at any time, said Klingbeil.

Kramp-Karrenbauer threw the CDU, and Merkel’s plan for a smooth transition of power, into turmoil on Monday with her announcement which followed months of mounting doubts about her suitability for the top job.

The last straw came last week when a local CDU branch defied her and voted with the far-right to install a local leader.