Rod Rosenstein is doing a star turn as principled defender of the law, but he’s performed abysmally as deputy attorney general, and President Donald Trump would be fully justified in firing him.

The leaked questions that special counsel Robert Mueller wants to ask Trump in a prospective deposition are, if accurate, a sign that Mueller has spun out of control on Rosenstein’s watch.

The questions (drafted by Trump’s legal team after consultations with Mueller’s investigators) suggest a free-floating investigation of the president’s motives, undertaken by a subordinate of the president. This is unlike any special counsel investigation we’ve ever seen and represents a significant distortion of our system.

Per the questions, Mueller wants to know how Trump reacted to news stories in The Washington Post. What he thought of FBI Director James Comey during the transition. How he feels about his attorney general.

These questions grow out of an obstruction-of-justice probe centered, as far as we can tell, on Trump’s exercise of the legitimate powers of the presidency. Mueller is out to prove that Trump had ill intentions. But this is an inherently problematic inquiry that involves a subordinate second-guessing the president on highly political questions.

It is doubtful that a president can be guilty of obstruction of justice in exercising his official duties, precisely because passing judgment on the lawful acts of a president is not a matter for prosecutors or the courts, but for the political process (i.e., for impeachment if the acts are deemed abuses of power). It’s another matter if a president has engaged in actual criminal conduct, like suborning perjury, but there is no indication of that.

What makes Mueller different from previous special counsels is that his predecessors were given the mission of investigating specific alleged crimes. As my National Review colleague Andrew McCarthy has repeatedly pointed out, Rod Rosenstein mentioned no crimes in his initial order to Mueller, a violation of the special counsel regulations. He said only that Mueller should investigate collusion and anything related.

This amorphous, wide-ranging guidance appears to have allowed Mueller to effortlessly slide from an amorphous, wide-ranging investigation into Russian meddling into an amorphous, wide-ranging investigation into obstruction of justice. (Rosenstein followed up later with a more specific memo to Mueller.)

Now, judging by the leaked questions, obstruction is the lion’s share of Mueller’s work. Absent smoking guns that we aren’t aware of (always possible), this is bizarre and disproportionate.

We now have an extensive obstruction investigation carried out by investigators who haven’t been obstructed. There’s been an intense focus, for instance, on Trump’s Oval Office discussion with then-FBI Director Comey about going easy on Michael Flynn. But as Andy McCarthy also notes, no one went easy on Flynn, who pled guilty to lying to the FBI.

Regardless, current Justice Department guidance says the president can’t be indicted. If Mueller takes heed, he is limited to indicting underlings and writing reports on his findings, with Congress the most important consumer.

This means Mueller is, in effect, the lead investigative counsel for a prospective House impeachment committee. It’s an important position, just not one that should be housed within the executive branch.

Rod Rosenstein is ultimately responsible for the state of this investigation. On the merits, he should be fired and replaced by someone willing to exercise proper oversight of the special counsel.

A more practical lever would be to push for Rosenstein to recuse himself. As a party to the firing of James Comey, he shouldn’t be overseeing a probe in which he’s a witness.

To this point, the White House posture toward the Mueller investigation has been to cooperate and hope it goes away, when a root-and-branch legal and constitutional challenge to Mueller’s work is now what’s called for.

Surely, Mueller will want to ask questions about such an effort, too — because he’s the unbounded investigatory ombudsman of the Trump era.

Rich Lowry can be reached via e-mail: comments.lowry@nationalreview.com.