It's a strange wrinkle in the German political system, but the fraught climate and the importance of this September's election make ironing it out more urgent than ever, according to two Bavaria lawyers.

Nuremberg law team Rainer and Christine Roth argue that the pact between Angela Merkel's conservative party, the Christian Democratic Union (CDU), and its Bavarian cousin, the Christian Social Union (CSU), deprives of them of their constitutional right to a free vote - since the CSU has been pushing a more right-wing agenda and pressuring Merkel - not least over her refugee policy.

In the face of this, the long-time conservative supporters say the issue has become critical - even though the alliance between the two parties has been in place since their birth in 1950. For that reason, the right to vote for a stabilizing centrist like Merkel is vital.

"The pressure has increased significantly in the year 2016 going into 2017," Rainer Roth told DW. "Not only domestically, but also because of the international political situation. The rise of two presidents who have to be treated with care leading the world's two superpowers, a lot of very difficult presidents of mid-sized powers, and we have a possible collapse of the European Union. We also have populist tendencies in Europe, including in Germany."

"In my opinion, we need universally-recognized politicians in this difficult time who can help shape Germany's affairs. And at the moment I don't see any alternative to Mrs. Merkel," he said.

Constitutional issue

But Rainer Roth can't vote for Merkel's party because Germany's electoral law is organized by state. As well as directly-elected candidates in individual seats, German parties field lists of candidates who are elected to the Bundestag according to the proportion of the national vote. But state party organizations draw up these lists - which means the CSU effectively functions as the CDU's regional representative, so they draw up the list from their own candidates.

As a solution, the Roths suggest that either the CDU be forced to field candidates in Bavaria, or, more realistically, that parties be allowed to set up national lists so that Bavarian CDU supporters can cast their vote for Merkel's party nationally.

But so far the Roths have failed to get a German court to agree with them. A court in Hesse dismissed their case last week, on the grounds that there was no legal basis to allow their suit, since there was no such thing as a national list, and it did not see that their constitutional rights were being damaged.

But Rainer Roth has not lost heart. "I think they missed the issue, because I want to know by which law my right to vote for a major party, a successful party after all, in the German Bundestag, can be limited," he said. "And this question wasn't answered."

Parties keeping quiet

Unsurprisingly, the two parties have kept quiet on the issue so far, since they both have a tactical interest in the status quo - the arrangement currently gives the CSU national influence with three ministers in Merkel's cabinet, and virtually assures the CDU victory in conservative Bavaria.

But over the past year, CSU leader and Bavarian state premier Horst Seehofer, facing a threat from the populist right-wing Alternative for Germany (AfD), has been laying down increasingly stinging ultimatums to Merkel, usually involving a cap on refugee intake, and the fissures between the parties have widened. This, as far as Rainer Roth is concerned, makes it "absurd" that he should be forced to vote for the CSU just because he lives in Bavaria.

For Frank Bösch, political historian at the University of Potsdam, this is the real value of the lawyers' suit - to point out the problematic relationship between the two parties. "They are not competing organizations, but directly bound together, through common organizations like the [youth organization] Junge Union," he told DW. "That makes the CSU formally a state association of the CDU. And yet its status as an independent party with a fixed parliamentary faction gives the CSU disproportionate political weight - even if the CDU's state association in North Rhine-Westphalia represents significantly more voters."

Thomas Schlemmer, CSU specialist at Munich's Institute for Contemporary History (IfZ), is sympathetic to the Roths' political frustration, but thinks their campaign is doomed, since the problem is so deeply rooted in Germany's federalist history and the cultural history of the different regions.

Not only that, he thinks the idea that a court might force a party to field candidates in parts of the country it didn't want to almost sounds "totalitarian." He also wonders whether the two parties really are so far apart as Seehofer's rhetoric sometimes makes it appear - after all, Merkel's asylum policy has largely followed the CSU's stricter course over the past year.

Rainer Roth has also initiated on online petition for his cause, which at time of writing had collected several hundred signatures. "The support from citizens is enormous," he said. "Today I've just been busy answering emails and taking phone calls."