There is little to mark the border between Germany and Austria at Kiefersfelden besides a European Union flag and three bored German policemen checking incoming cars for migrants. But this may be where things started to unravel for Angela Merkel’s Bavarian allies.

Bavaria votes on Sunday in regional elections that will be the latest to shake the established order in Europe. Mrs Merkel’s regional sister party has ruled Bavaria since 1957, but if the polls are right, the Christian Social Union (CSU) faces a fight to hold onto power.

For once, the threat is not a populist revolt. Instead, the CSU appears to have self-destructed in its attempts to head off a challenge. Horst Seehofer, the CSU leader, almost tore Mrs Merkel’s government apart over the summer.

Across the Alps from Kiefersfelden lies the Brenner Pass between Austria and Italy, where experts were predicting a new wave of migrants would surge north towards Germany.

Fearing that would play into the hands of the nationalist Alternative for Germany party (AfD), Mr Seehofer demanded the right to turn back migrants at the border. If he didn’t get his way, he threatened to resign as interior minister, pull the CSU out of Mrs Merkel’s coalition and deprive her of a majority in parliament.

But the new wave of migrants has not materialised and instead of safeguarding its vote, the CSU has plummetted in the polls.