The relevant statute, Section 1512(c) of the federal criminal code, applies, as Mr. Barr says, to cases of evidence impairment, but it also applies to anyone who “otherwise obstructs, influences, or impedes any official proceeding, or attempts to do so” — provided that they act “corruptly.” If destroying evidence to protect oneself from an investigation is obstruction, then so is pressuring a subordinate to ignore such evidence or drop the investigation altogether.

Mr. Barr argues that President Trump cannot be criminally liable under Section 1512(c)(2) because “statutes that do not expressly apply to the President must be construed as not applying to the President if such application would involve a possible conflict with the President’s constitutional prerogatives.” If Mr. Barr were right about this, then the president would be shielded from a host of uncontroversial laws, including the federal bribery statute, which does not mention the president but would seem to prevent him from selling a cabinet post to the highest bidder.

Mr. Barr also says that the obstruction statutes do not apply to “facially lawful” acts by the president such as the firing of an F.B.I. director, because presidents are constitutionally authorized to fire their subordinates. But the obstruction statutes do apply to actions that would be “facially lawful” under other circumstances. For example, there is no law against tearing up pieces of paper; there is a law against tearing up documents so that they cannot be subpoenaed by federal prosecutors. Firing the F.B.I. director is not a crime; firing the F.B.I. director in order to block an investigation into the president’s own actions very well might be.

Finally, Mr. Barr says that the president’s motive in removing James Comey as F.B.I. director “could not have been ‘corrupt’ unless the President and his campaign were actually guilty of illegal collusion.” Nonsense. Obstruction is a crime in itself, and for an obvious reason. If a criminal suspect successfully obstructs an investigation, the underlying crime is never discovered.

Twenty years ago, a bipartisan group of four former attorneys general warned that attacks on the independent counsel Kenneth Starr’s investigation of President Bill Clinton “by high government officials and attorneys representing their particular interests” appeared to have the “improper purpose of influencing and impeding an ongoing criminal investigation.” Mr. Starr — who was “effectively prevented from defending himself” because of confidentiality considerations — should be allowed to continue his work, the former attorneys general said, “without harassment by government officials and members of the bar.” Wise words, which apply to the current special counsel as much as they did to Mr. Starr. And Mr. Barr would do well to remember that he wrote those words himself.