BURLY, jovial and sonorously rolling his Rs, Horst Seehofer, the boss of the Christian Social Union (CSU), embodies Bavarian populism and bravado. The CSU is a party that only exists in Bavaria, one of the richest, most successful and most distinctive of Germany’s 16 states. It has governed there for almost all of post-war history, often with an absolute majority, as now with “King Horst” as state premier. But its power extends deep into national politics, making it often more volatile and always colourful.

Mr Seehofer is best observed working the crowd in a bustling beer tent, all ears for the popular mood, building up his “coalition with citizens”. With sly innuendo against the bureaucrats of Brussels or Berlin, against immigrants milking the welfare system or foreign drivers clogging motorways, he can excite an audience even without any clear ideology or policies.

With a model railway taking up his basement, his hobbies and tastes are reassuringly ordinary. Even revelations about an illegitimate daughter have not hurt him, or his party’s advocacy of family values, in a state that lives by a relaxed Catholic brand of live-and-let-live. A Hund is er scho’ (Sure, he’s a dog, in Bavarian dialect) is a phrase often applied to him. It expresses the macho cocktail of frisson and admiration that is necessary for success in Bavaria, says Werner Weidenfeld, the director of the Centre for Applied Policy Research in Munich. And the CSU is indeed, he says, “the most successful party in Germany”.

Among regional parties it is unique in the world. Scots, Catalans and Québécois may have parties representing them, but none has the CSU’s combination of regional and national power. That is thanks to an arrangement with its sister party, the Christian Democratic Union (CDU), led by Angela Merkel, Germany’s chancellor. When the two parties emerged in the aftermath of the second world war, they agreed that the CDU would never enter Bavaria, nor the CSU the other German states, but that both would form one group in the national parliament. The CSU has never fielded the chancellor (though it tried in 1980 and 2002). But its strong election results in Bavaria have buttressed every CDU chancellor from Konrad Adenauer to Mrs Merkel.

Both parties are “Christian unions” because they united Catholic and Protestant parties that dated back to the Weimar Republic. They absorbed liberals and conservatives as well. The CSU also emphasises the “S” in its name, for “social”, which in Germany stands for workers’ interests, class solidarity and some redistribution. And it swallowed most of what would have become a Bavarian-independence party. By integrating so many different parts, it has prevented the fragmentation and extremism that were the bane of the Weimar Republic, says Mr Weidenfeld.

The CSU owes its success to the long transformation of Bavaria from agricultural backwater to industrial and high-tech powerhouse without loss of local identity. The proud term for this combination is “laptops and lederhosen”. And in every campaign—municipal elections are on March 16th—the CSU mobilises voters with the spicy, cheeky language of Bavarian patriotism. Thus half of Mr Seehofer’s act is to cause controversies in Berlin or Brussels so that his Bavarians know he is looking out for them. The other half is to drop those controversies quickly to ensure no lasting harm to Mrs Merkel or the country.

To his detractors that is irresponsible flip-flopping. For example, Mr Seehofer is in favour of Germany’s (popular) plan to shift from nuclear and fossil-fuel energy to renewable sources. But he simultaneously panders to those Bavarians who oppose new and ugly power lines to bring electricity into the state. “He gets them up the tree but how do you get those folks down again?” complains Margarete Bause, the leader of the Green party in Bavaria’s state parliament. “With his populism he endangers the entire energy transition.”

Neither Mr Seehofer nor his CSU is as dangerous as that, says Heinrich Oberreuter at the University of Passau. That is because the CSU knows two things: first, that the CDU would not govern Germany so often without Bavaria’s CSU; and second, that the CSU also needs Mrs Merkel’s popularity to do well in Bavaria. So the CSU must make Mrs Merkel’s life difficult; just not so difficult that she fails.