Senior officials at the Justice Department have claimed that executive privilege should bar a lawyer dismissed from the department from testifying to Congress about a disagreement with supervisors over restoring the gun rights of Mel Gibson, the actor and prominent supporter of President Trump.

Nevertheless, Elizabeth G. Oyer, the department’s former pardon attorney, appeared before lawmakers on Monday afternoon. “I am here because I will not be bullied into concealing the ongoing corruption and abuse of power at the Department of Justice,” she told Democratic senators and representatives.

The dispute began on Friday night, when a lawyer in the office of the deputy attorney general warned Ms. Oyer that she was “not authorized to disclose” records about the firearms-rights issue to lawmakers, according to a letter reviewed by The New York Times.

A lawyer for Ms. Oyer responded in kind, accusing the department of trying to intimidate a whistle-blower on the cusp of a congressional hearing.

The department’s argument that internal conversations are covered by executive privilege — a legal doctrine meant to keep some discussions in the executive branch off-limits to the other branches of government — could have consequences for many other former Justice Department lawyers. Already, the department has fired dozens of career prosecutors, some of whom have spoken publicly about their experiences, while others may yet do so.

On the same day that the department informed Ms. Oyer of its plans, she also learned that deputy U.S. marshals would be sent to her home to deliver the department’s letter. After Ms. Oyer assured officials that she had received it via email, the delivery was canceled.

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