The Christian Social Union (CSU) is not usually a party to break with tradition. But at this weekend's annual party conference in Munich, the Catholic conservative party dominated by Bavarian patriots has abandoned decades of protocol and decided not to invite the leader of Germany's main center-right force, the Christian Democratic Union (CDU), to make her usual guest appearance.

Chancellor Angela Merkel's absence was played down by both allies when it became public last week. "It's a mutual decision of the two party leaders, following the motto: togetherness instead of staging," said CSU General Secretary Andreas Scheuer. "We've always said the issues should be in the foreground."

But there's little doubt that there may have been another reason why Merkel and CSU leader Horst Seehofer might not be keen to share a stage this time - few can forget the humiliation that befell the chancellor at last year's conference, when, immediately after the speech in which she again refused to offer an upper limit on the number of refugees allowed to enter Germany, Merkel was forced to hang around on stage while Seehofer voiced his party's objections. "We'll meet again on this subject," he told her.

Both the CDU and the CSU are keen to avoid a similar scene this year, because there is still daylight between the two allies on the "upper limit" issue a year later. "There's no point clearing up open questions on stage," CSU deputy leader Ilse Aigner told the "Welt am Sonntag" newspaper. "We saw last year what it looks like when you celebrate disagreement. There's no need to repeat that."

A regional national party

The CSU is Germany's most paradoxical major political party. Identifying staunchly with the "Free State of Bavaria," the CSU always aspires to - and usually gets - federal power (the party currently boasts three cabinet ministers, covering the agriculture, transport, and economic development briefs in Merkel's cabinet). 

The reason for this is the CSU's long-term alliance with the CDU. The two parties have been locked together by a pact sealed at the dawn of the Federal Republic of Germany that has benefitted both parties - in exchange for not fielding any candidates in Bavaria, the CDU is guaranteed the support of the CSU on a national level. But occasionally, the pressure of national crises creates fissures.

The past year has been one of those times. An influx of several hundred thousand refugees into Germany created a bureaucratic impasse in many parts of the country, and handed media attention and then regional electoral success to the Alternative for Germany (AfD), an anti-EU party that swiftly changed gears and capitalized on fear and xenophobia by offering straightforward anti-immigrant, anti-government populism.

Threat from both sides

In more harmonious times, Merkel and Seehofer seemed happy for Seehofer to play the role of right-wing attack dog to Merkel's studied, cautious centrism - the formula worked with Germany's conservative voters, and allowed the CSU to stick to the famous policy dictum expressed by its most storied leader, Franz-Josef Strauss: "There must be no democratically legitimate party right of the CSU."

But with the AfD gaining nearly a quarter of the vote in some German states this year, that motto has become obsolete. The CSU is currently polling at only 44 percent in its home state, while the AfD has taken 9 percent - anything less than an absolute majority was once considered unthinkable.

And it gets worse for the CSU - the split in the right-wing vote has opened opportunities for a so-called "red-red-green" coalition. Germany's three big leftist parties - the Social Democrats (SPD), the Left party, and the Greens - have suddenly sensed an opportunity and found common ground. That particular power constellation is about to take over in the state of Berlin, and is being seen as a blueprint for an alternative to a Merkel-led "grand coalition" following the general election in September 2017.

"We're in a two-front war," former CSU leader Erwin Huber told the "Frankfurter Allgemeine Zeitung" on Friday. "Red-red-green on the one side - that formation is now a serious prospect. If the Union isn't strong enough, then that - I'm convinced - will come after the Bundestag election. And on the other side we have the right-wing populists. Germany is in flux and we're in the pincers of the party system."

That might well be why the CDU and the CSU are now so keen to cover the cracks in their relationship. Recently, prominent CSU politicians have spoken with renewed enthusiasm about the prospect of having Merkel as the CDU/CSU candidate next year - for the fourth time running.

"It has become very clear [at the recent two-party summits] that there are huge areas of agreement between the CSU and the CDU," CSU parliamentary leader Gerda Hasselfeldt told the DPA news agency. "That's why I'm very confident that we'll be fighting together next year, election year, together." With or without Angela Merkel.