Horst Seehofer, who leads the Christian Social Union (CSU) - the Bavarian sister party to Merkel's Christian Democratic Union (CDU) - told the Süddeutsche Zeitung that “the situation is highly threatening.”
In comments published on Tuesday, two days after the AfD won 21 percent of the vote in Mecklenburg-Western Pomerania, Seehofer claimed people “don't want this Berlin politics” - in reference to the government's liberal refugee policy.
The Bavarian leader said that he had “repeatedly” called on Merkel to change her policy on refugees but that she had not listened. The “disastrous” result in Mecklenburg-Western Pomerania, where the CDU won only 19 percent of the vote, was a consequence of this, he stated.
Seehofer called on Merkel to make clear a comprehensive plan for winning back support that would include “taxes, internal security, pensions, migration - it needs to be finished by the latest in September or October.”
Merkel has yet to announce her candidacy for re-election as Chancellor in national elections in 2017. There has been speculation in the media that she is delaying the announcement because she has yet to gain the backing of Seehofer.
The CSU are more conservative in their politics than the CDU, which has a presence in every German state other than Bavaria, the wealthy and populous southeastern state where Munich is capital.
Soon after refugees started arriving in Germany in large numbers in the second half of 2015, the CSU started calling on Merkel to impose restrictions such as an upper limit on the number of refugees arriving.
Since two Isis-related attacks took place in July in Bavaria, which led to dozens of injuries and the deaths of both attackers, the CSU has renewed calls for tougher asylum policies including sending people back to war zones.
Critics have accused the CSU of contributing to the rise of the AfD by using populist, xenophobic rhetoric and proposing policies which are far to the right of the political mainstream.