Then-special counsel Robert S. Mueller III makes a statement on the investigation into Russian interference in the 2016 presidential election on May 29 in Washington. (Salwan Georges/The Washington Post)

FORMER SPECIAL counsel Robert S. Mueller III already had made clear that he would not say much beyond what is in the report he released in March when he testifies to House committees Wednesday. So the “guidance” the Justice Department sent him on Monday warning him not to stray from what is already in the public record may have little effect on his testimony . In other words, the Trump administration’s last-minute attempt to bully Mr. Mueller into silence is superfluous, as well as offensive.

But let’s not lose sight of the offensiveness. An administration that routinely flouts Congress’s legitimate inquiries into alleged misconduct by some of its highest officials now has the gall to propose limits that it has no right to impose. These must not be accepted as precedent for what those who have investigated a president can and cannot say.

Justice Department policy, the guidance noted, precludes discussing “the conduct of uncharged third-parties” — that is, President Trump — as well as “any comment on the facts developed and legal conclusions by the Special Counsel’s Office with respect to uncharged individuals.” The guidance also warned that additional material, including “investigative steps or decisions made during your investigation,” might be covered by various forms of privilege.

In fact, Mr. Mueller faces few true legal restraints. As a former Justice Department employee, he cannot reveal classified information or sealed grand jury material. But he could freely elaborate on, say, why he decided neither to accuse the president of a crime nor to exonerate him, a call that confused many and remains a point of public disagreement.

There are ethical considerations former prosecutors must consider when releasing information on the behavior of people who were investigated but not indicted. But Mr. Mueller’s situation is unusual. According to the Justice Department, a sitting president cannot be indicted. According to this thinking, the most direct remedy for a law-breaking president is impeachment by Congress. Yet if investigators are barred from sharing information with lawmakers, who take on a quasi-judicial role in the impeachment process, there might be no effective remedy at all.

Mr. Mueller should feel free to share material that could aid Congress in determining whether to proceed with an impeachment inquiry. So should future special counsels investigating future presidents.

Nor should Mr. Mueller hesitate to discuss the spin job that Attorney General William P. Barr performed around the time of the report’s release. If the attorney general misrepresented his conversations with Mr. Mueller to protect the president, or if Mr. Mueller believes the attorney general mischaracterized the report, the public deserves to know.

Finally, it should not be an afterthought — but it so often is — that the nation’s democracy was attacked by a hostile foreign government during the 2016 presidential election. To the extent Mr. Mueller has anything to add that could spur lawmakers into more forcefully addressing the continuing threat, he should share that, too.

It is not Mr. Mueller’s duty to remain silent for the comfort of the president.