President Trump now says he "would love to" be interviewed under oath by special counsel Robert S. Mueller III. The news comes amid signs that Mueller's investigation is moving ever further into the president's inner circle. An interview with Mueller or testimony to the grand jury would be new and potentially dangerous ground for the president. His lawyers and allies may well have reason to fear the president would not be truthful, which may be why some are trying to disparage the interview before it even takes place.

In recent days, some of the president's allies have floated the idea that Mueller's interview could be a "perjury trap." The president's lawyer, Ty Cobb, said last week that he hoped the process would be fair and not simply a perjury trap. Roger Stone, a longtime informal adviser to Trump, told The Post that the president would be foolish to agree to such an interview: "Why would you walk into a perjury trap?"

The word "trap" connotes a snare set by investigators for the innocent and unwary. Should the president end up charged with perjury or false statements, you can expect to hear arguments that the charges are illegitimate because Mueller unfairly caught the president in a trap. Or perhaps the president and his lawyers will use the perjury-trap claim to justify refusing to be interviewed altogether.

But "perjury trap" is a specific legal defense, related to entrapment. A claim of a perjury trap is really a claim of prosecutorial misconduct. It refers to an abuse of the legal process, whereby a prosecutor subpoenas a witness to testify not for a legitimate investigative purpose but to try to catch him in an inconsistency or falsehood — even a relatively minor one — that can then trigger a perjury charge.

(Strictly speaking, and despite the president's offer, an interview with prosecutors and the FBI typically would not be under oath, so perjury would not apply. But whether the president is under oath or not makes little practical difference. Lying during an unsworn interview is still a crime: false statements, the same offense to which Michael Flynn and George Papadopoulos pleaded guilty. Either way, the "trap" argument is the same — the claim that investigators interviewed the president only to try to catch him in a lie.)

No objective observer could claim Mueller does not have a legitimate investigative reason to interview Trump. Trump was, of course, the head of his campaign and is therefore a key witness on all issues related to possible Russian interference and collaboration. And if Mueller is probing possible obstruction of justice by the president himself, it is critical to try to get the president's side of the story. It's entirely appropriate and expected for Mueller to seek the president's testimony.

Being called to testify and therefore having the opportunity to commit perjury does not make that testimony a perjury trap. If that were the case, every witness could make the same argument. Claiming that Trump's testimony would be a perjury trap is like saying driving a car is a "speeding trap" because being behind the wheel gives you the opportunity to exceed the speed limit.

Characterizing the president's interview as a potential perjury trap is simply wrong. But it is of a piece with the broader effort by the president and his political allies to discredit Mueller's investigation. It suggests — wrongly — that Mueller is treating the president unfairly. If the president commits perjury or false statements, it will be because he chose to lie — not because he was caught in a "trap."