German Chancellor Angela Merkel | Zick Jochen/Pool for Getty Images Top German politician: Merkel’s days are numbered ‘Angela Merkel has clearly reached her peak,’ deputy president of the Social Democrats said.

Leaders from Germany's conservative and center-left parties have stepped up criticism of Angela Merkel in the wake of a stinging loss to the anti-immigrant Alternative for Germany (AfD) party in the chancellor's home state.

"Angela Merkel has clearly reached her peak," Ralf Stegner, a deputy president of the Social Democrats, a coalition partner of the chancellor's Christian Democrats (CDU), told Spiegel Online in an interview published Tuesday.

Merkel's party finished third in Sunday’s poll in the northeastern state of Mecklenburg-Vorpommern, falling behind the far-right AfD for the first time in one of the party’s worst showings since World War II.

Stegner said that after the disastrous regional election, Merkel may be forced to disqualify herself from the race to be chancellor at next year's election, stepping aside in favor of Horst Seehofer, the head of CDU's Bavarian sister party, the Christians Social Union (CSU).

"Historically, the CDU has always been pitiless towards chancellors when it had the impression that there could be a massive loss of seats," Stegner said.

Seehofer, for his part, was quick to lay blame for the conservatives' poor showing in the regional election on Merkel.

His "repeated calls for a change of course" had been ignored ahead of the election, which led to the "disastrous" results, Seehofer told Süddeutsche Zeitung. Merkel's refugee policy, which many within the chancellor's camp have blamed for her falling popularity, is "only an outlet, the problems go much deeper," he said.

Merkel took responsibility for her party's poor election results, saying on Monday: "I am the head of the party and chancellor … naturally I am also responsible." But she stood by her decision to accept more than one million refugees in Germany last year.