BERLIN — Horst Seehofer, the defiant German interior minister whose provocations have undermined Chancellor Angela Merkel’s government and eroded support for his own conservative party, said Monday that he would step down as head of the Christian Social Union, a key part of Ms. Merkel’s governing coalition.

Mr. Seehofer’s announcement that he plans to retire from party leadership, although he said he would remain in his more powerful government post, comes two weeks after Ms. Merkel shocked the country by announcing that she would do much the same thing. The Christian Social Union, which exists only in Bavaria, is the sister party to Ms. Merkel’s Christian Democrats, who operate across the rest of the country.

“I’m the federal interior minister and will continue to exercise my duties,” Mr. Seehofer said. His government job, he told reporters, “is in no way affected” by his decision.

Mr. Seehofer, 69, has recently faced calls to step down from his government post, and he has been under pressure within his own party since it suffered severe losses in a state election last month. He had hoped to rally conservatives around him, but instead earned a reputation for reckless political moves.