Bavarian leader Horst Seehofer escalated a dispute with Chancellor Angela Merkel on Tuesday, threatening in a letter to contest her immigration policy in court unless she changes course.

“This development can’t be allowed to continue,” Seehofer wrote, arguing that Merkel had a constitutional responsibility to protect his state and others from “uncontrolled” immigration.

More than a million people applied for asylum in Germany last year, a tenfold increase from 2013. Bavaria, situated on the country’s southern border with Austria, has been bearing the brunt of the refugee flow.

While Seehofer, who leads Merkel’s Bavarian sister party, the Christian Social Union (CSU), has publicly disagreed with the chancellor’s migration policy before, his threat to sue the federal government is highly unusual.

The Social Democrats, the third party in Merkel’s government, were quick to exploit the disunity on the right, calling Seehofer’s letter “a declaration of a break with the coalition.”

“One doesn’t write threatening letters in a coalition, one solves problems,” said Thomas Oppermann, the Social Democrats’ leader in parliament.

Politics as usual?

The left is keen to portray itself as the more responsible of the three ruling parties with an eye toward the next election.

Political calculation may help explain Seehofer’s move as well.

The Economist recently cautioned against reading too much into the Bavarian’s disloyalty. “The CSU has always been prone to elaborate displays of dissent,” the newspaper said — “without which it would have no reason to exist as a separate party from the CDU,” Merkel’s Christian Democratic Union.

The CSU has three jobs. The first is to rule Bavaria, “which it does competently.” The second is to ensure that no legitimate party ever emerges to the right of the Christian Democrats, who must be “populist enough to appeal to conservative voters and keep them from drifting to the extreme right.”

The third job is to make enough trouble in national politics, especially for the CDU, for Bavarians to feel important — but without actually toppling a Christian Democrat chancellor (without whom the CSU would also be powerless).

Perhaps Seehofer is just following the script. But this time seems more serious.

Overwhelmed

The enormous influx of immigrants in the last year has overwhelmed the German immigration authorities as well as the cities and towns who must shelter them.

Merkel still insists that the country can “manage,” but public sentiment has turned against her in the wake of attacks by foreigners in Cologne and other German cities on New Year’s Eve.

Hundreds of women and girls were intimidated and robbed that night by groups of men of Arab and North African appearance. Several women have said they were raped.

Merkel responded to the attacks by extending judges’ power to deport migrants convicted of serious crimes.

Other countries have gone further. Austria “temporarily” suspended its Schengen free-travel arrangements with other European nations this month, arguing that immigration must be “controlled” and that economic migrants ought to be sent back. Denmark, Hungary and Slovenia have taken similar steps.

Polls show most Germans now favor capping the number of immigrants, something Seehofer has advocated for months.

Meanwhile, the anti-immigrant Alternative für Deutschland party keeps rising the polls. It would now overtake the Greens as Germany’s third largest.

If worse comes to worst, the CDU and the Social Democrats have enough seats in parliament to govern without the CSU. But a split in the conservative union would be disastrous for Merkel and almost certainly prompt call for new elections, if not her resignation.