REUTERS • GETTY Angela Merkel is set to be snubbed by her coalition allies

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The German Chancellor's house of cards is crashing down around her after the Christian Social Union (CSU) - her key coalition partners - remarkably threatened to pull the plug on co-operation. In an unprecedented political humiliation, the party said it would not take part in a keystone congress event unless Mrs Merkel immediately reversed her derided open door migration policy. It portrayed the German leader as arrogant, elitist and out of touch with ordinary people, branding her infamous "wir schaffen das", meaning we can do it, slogan patronising to ordinary voters.

The sight of the lone Chancellor taking the stage without her crucial partners in government would be a hugely symbolic moment, demonstrating how the effects of mass migration have pushed the unity of the German establishment beyond even its limit. It would constitute a devastating attack on the beleaguered German leader and caps off an extraordinary week which has seen her reputation as a competent leader broken beyond repair. Nerves have been growing in Berlin after Mrs Merkel's Christian Democratic Union (CDU) lost out to far-right activists in her hometown and polling showed the party will also take a battering in the capital later this month.

AFP Bavaria state premier Horst Seehofer has called on the German leader to reverse her migration policy

GETTY Mass migration has fuelled a far-right resurgence in Germany

In particular there has been anger amongst the CSU, which fears it is being tarnished by Mrs Merkel as voters view open door asylum as a policy of her entire government. Voters have been turning in their droves to the far-right Alternative for Deutschland (AfD) party, which is set to scoop seats in 10 of the country's 16 regional assemblies despite only forming in 2013. And now the CSU leader has been forced to act, vowing to make a symbolically defiant stand against a drive towards mass migration which has played into the hands of the country's resurgent far-right. Bavaria state premier Horst Seehofer threatened to skip the CDU's annual congress in December, dealing an unprecedented blow to the unity of the conservative bloc nine months before next September's election.

It doesn't do any good for people to keep saying 'we've done everything right, you just don't understand it' Bavaria state premier Horst Seehofer

He blasted: "It doesn't do any good for people to keep saying 'we've done everything right, you just don't understand it'. "That's only going to make the AfD stronger." Mr Seehofer repeated his demand that the German Chancellor bring in upper limits on the number of refugees arriving in the country, something she has repeatedly refused to contemplate. He said: "There can't be any compromise about that. Without reaching a consensus on that, my appearance at the CDU congress would be nothing more than a media spectacle." Even though the CDU and CSU traditionally agree on a joint candidate to run for chancellor, Mr Seehofer has not yet endorsed Mrs Merkel for next autumn's general election.

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