German Chancellor Angela Merkel has accepted a two-week deadline over tougher migration policy set by Interior Minister Horst Seehofer, who leads her Bavarian coalition partner, according to a person familiar with the matter.

Seehofer will give the chancellor until the end of the month to strike a Europe-wide agreement over refugees before he takes unilateral action to reject certain migrants at the German border, which Merkel has opposed, said a person familiar with the situation, who asked not be identified ahead of a public announcements. Merkel and Seehofer are due to speak to the press at 2 p.m. local time.

Seehofer’s ultimatum has become one of the biggest challenges to Merkel’s authority since she took power nearly 13 years ago. The deadline shows how emboldened the chancellor’s Bavarian ally feels as Hungary, Italy and Austria adopt hard-line stances on protecting national borders. Even with a sharp decline in the numbers crossing to Europe over the Mediterranean, migration has surged to the top of the political agenda, with the proliferation of unilateral measures fraying the bonds of EU unity.

Merkel has lobbied for time to reach a broader deal and avoid border controls, which threaten to undermine the bloc’s principles of free movement. Migration is “a European challenge that requires a European answer,” Merkel said in her weekly podcast released Saturday. “I see it as one of the most decisive issues in holding Europe together.”

Failure to reach a deal that can be presented as a breakthrough at the European level could precipitate a full-blown crisis in Germany that might topple Merkel after almost 13 years as chancellor. Her demise would likely further bolster authoritarian governments in eastern Europe, undermine the new Spanish government’s humanitarian stance on migration and put at risk Macron’s plans for euro-area reform.