The president of the United States will not talk to special counsel Robert Mueller because his lawyers want to prevent him from walking — or, rather, talking — into a perjury trap.

Rudy Giuliani, the president’s most visible lawyer, insists that no attorney worth his license would permit a client to testify under such circumstances. For many of Trump’s detractors this is tantamount to a confession that the president would lie. Many even insist there is no such thing as a “perjury trap.”

That’s wrong. Former federal prosecutor Andrew McCarthy, my National Review colleague, notes that “for charging purposes, the witness who answers the questions does not get to decide whether they have been answered truthfully. That is up to the prosecutor who asks the questions.”

In other words, if Donald Trump testifies that he didn’t murder Col. Mustard in the library with the candlestick, and Michael Cohen says Trump did it, the prosecutor decides who is lying.

Presumably Mueller would consider other evidence too. Trump and Cohen are both legendary liars, after all. So Mueller would surely look to corroborate either’s testimony. If the candlestick bore Trump’s fingerprints, Mueller might believe Cohen. If Col. Mustard spelled out “It was Cohen” in his own blood, Mueller would probably side with Trump.

But the point remains: When it comes to charging someone, the prosecutor gets to decide who’s lying. For a conviction, however, it falls to a jury or judge to decide whether someone is truly guilty.

Now there are many problems with my analogy. For starters, no one is accusing Trump of murder. Also, as a matter of Justice Department policy and common constitutional interpretation, a sitting president cannot be indicted.

This fact illuminates one of the many flaws with Giuliani’s defense. He claims he’s protecting Trump from being criminally charged with perjury while also insisting the president cannot be criminally charged with perjury (or anything else).

If pressed, the former New York City mayor would probably say he’s also protecting Trump from criminal prosecution after he leaves office, or from impeachment.

But don’t let the phrase “high crimes and misdemeanors” fool you. Impeachment isn’t a legal process at all; it’s a political one. The impeachment power vested in Congress is for breaches of the public trust, and Congress has the sole power to decide what constitutes such a violation.

While it is possible that a Democrat-controlled House might vote to impeach Trump, it seems incredibly unlikely that a two-thirds majority of the Senate would vote to remove the president based on what we know now.

But my point here isn’t really to demonstrate the weaknesses of Giuliani’s legal and political arguments. Nor is it to point out that he zigzags between making legal arguments for political purposes and political arguments disguised as legal ones. Rather, it’s to illuminate the fact that while Giuliani is right to keep Trump from testifying, he’s not making an argument on behalf of the public interest. Like Tom Hagen in “The Godfather,” Giuliani has a special practice with only one client.

Giuliani is doing whatever he can to protect that client. Where he draws moral or ethical lines to that end is between him and his conscience. But the rest of us are not obliged to argue like we’re Trump’s legal water-carriers. Refusing to testify might make total sense as a matter of Trump’s personal self-interest — which is Giuliani’s concern — but it is not obvious that such a refusal would be in the public interest. We deserve to know all of the relevant facts. If that makes the president’s job harder, so be it. The Constitution makes every president’s job harder in myriad ways, but we don’t say we that should therefore get rid of the Constitution.

As Ramesh Ponnuru recently noted at Bloomberg.com, the president has special obligations, including a sworn duty to faithfully execute the laws and safeguard our national security. Denouncing or stymieing investigations into foreign meddling, encouraging convicted criminals not to snitch, opining on active trials and similar conduct are violations of his sworn duties.

Trump’s supporters — the ones who aren’t on his payroll — are free to make any argument they want in his defense, but they’d probably be more persuasive to the unconverted if they focused on the public interest and not the president’s self-interest.

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