BERLIN — President Christian Wulff of Germany fought back on Wednesday against a barrage of criticism over his handling of a private loan scandal, insisting in a nationally broadcast interview that he would not consider stepping down while admitting that he had made a “grave mistake.”

The uproar surrounding the president has threatened to engulf Chancellor Angela Merkel, who in 2010 handpicked Mr. Wulff for the presidency, elevating if also sidelining Mr. Wulff, a political rival, to a largely symbolic post. Calls for his resignation grew louder on Wednesday, with some members of Mrs. Merkel’s conservative party joining in with the opposition.

Mrs. Merkel made her first comment on the scandal ahead of the broadcast, expressing “full confidence” that Mr. Wulff would offer a sufficient explanation to all questions.

“The chancellor is absolutely confident that the president will continue to answer all open questions completely,” Mrs. Merkel’s spokesman, Georg Streiter, told reporters. “She has enormous respect for the work of the president.”