Annegret Kramp-Karrenbauer will step down as leader of Germany's CDU | Carsten Koall/Getty Images Merkel heir Kramp-Karrenbauer to step down as CDU leader She also won’t run for the chancellery.

BERLIN — Annegret Kramp-Karrenbauer plans to step down as leader of Germany's governing Christian Democrats and will not be the party's candidate to succeed Angela Merkel as chancellor, German media reported Monday.

Kramp-Karrenbauer, who was widely seen as Merkel's preferred successor as chancellor, informed the party of her decision at a CDU executive committee meeting on Monday morning.

The announcement came after an outcry over a controversial vote in the eastern German state of Thuringia last week. The regional CDU branch had acted against Kramp-Karrenbauer's wishes and voted with the Alternative for Germany (AfD) to install a liberal as state premier, the first time in Germany's postwar history that a state premier was elected with the help of the far right.

Kramp-Karrenbauer, who currently also serves as defense minister, won a vote to succeed Merkel as CDU leader in 2018, but has struggled to establish her authority within the party. The Thuringia vote further threw into doubt her ability to impose party discipline.

She said she would only remain as defense minister if her party and parliamentary group supported her, according to the German tabloid Bild.

Magazine Der Spiegel reported that she had mentioned the unclear positions of some in the CDU toward the AfD and the leftist Die Linke party as a reason for her resignation.

Kramp-Karrenbauer opposes any cooperation of her party with either the AfD or Die Linke. The latter is the largest party in the Thuringian parliament; Linke politician Bodo Ramelow was expected to win a second term as state premier and lead a left-wing minority government.

But on Wednesday, the AfD abandoned its own candidate for the premiership and voted alongside the CDU and the pro-business FDP to elect liberal Thomas Kemmerich in order to prevent Ramelow from winning. The vote triggered a nationwide outcry, with many warning that a taboo on cooperating with the far right had been breached. Kemmerich resigned on Saturday.

The Frankfurter Allgemeine Zeitung reported that Kramp-Karrenbauer would stay on as leader until the party elects a successor this summer. AKK, as she is often called, was expected to continue the CDU's centrist course under Merkel, who is planning to step down as chancellor at the next federal election, currently scheduled for next year.

Potential candidates for the CDU leadership include Health Minister Jens Spahn and Merkel critic Friedrich Merz, who both lost against Kramp-Karrenbauer in 2018, as well as North-Rhine Westphalia state premier Armin Laschet.

Spahn on Monday expressed "great respect" for Kramp-Karrenbauer's decision and urged party "cohesion."

Annalena Baerbock, co-leader of the Greens, warned of a potential "power vacuum" and said all parties must focus on "upholding a clear-cut firewall against the AfD."

She told the German news agency DPA that the CDU, which governs in a coalition with the Social Democrats, also needed to "clarify how it can lead a stable government under these conditions." The Greens are polling in second place behind the CDU.

Wolfgang Schäuble, a senior CDU figure and president of the Bundestag, expressed fear for the future of his party. "If we continue like this, [our] next chancellor candidate won't become chancellor," he told Bild.