Donald Trump shouldn’t fire Robert Mueller, but Mueller should be the last special counsel.

The Mueller probe just took a Ken Starr turn with its lurch, via the Southern District of New York, into the Stormy Daniels affair.

After the Starr investigation in the 1990s, there was a consensus that we weren’t doing that again, certainly not through the independent counsel statute, which lapsed. The law put investigations on a hair trigger and carved out independent counsels, executive branch officials, from control of the chief executive in a constitutionally impermissible way. Endless politically fraught investigations ensued that often exhibited a zeal disproportionate to the alleged crime.

It’s too early to render a verdict on Mueller’s work, but he certainly appears to have become a kind of free-floating legal ombudsman.

In response to Trump’s blustery attacks on Mueller, a bipartisan group of lawmakers is calling for legal protections against Mueller’s removal that would be an ill-advised step back toward the independent counsel statute. Instead, we should be thinking of whether this is the best way to hold presidents accountable in the future. As a practical matter, it’s hard to imagine any administration ever permitting such an investigation to get unloosed again.

Even if Trump is fully vindicated, the probe has exacted a significant price, and much of the left considers the Mueller probe a resistance march with subpoena power.

In his famous dissent in the Supreme Court case of Morrison v. Olson upholding the independent counsel law in 1988, Antonin Scalia wrote, “Nothing is so politically effective as the ability to charge that one’s opponent and his associates are not merely wrongheaded, naive, ineffective, but, in all probability, ‘crooks.’ And nothing so effectively gives an appearance of validity to such charges as a Justice Department investigation and, even better, prosecution.”

This is why each side celebrates when it can get such an investigation going — and they know it will ramify in unpredictable, harmful ways.

Scalia quoted FDR’s attorney general, Robert Jackson, who warned against prosecutors picking a person to investigate rather than a crime, lest it become a matter of “searching the law books, or putting investigators to work, to pin some offense on him.”

Now, obviously, Deputy Attorney General Rod Rosenstein, a Trump appointee, didn’t intend to create such a dynamic (although his initial mandate to Mueller was much too broad). And Trump brought this all on himself with his ham-fisted firing of FBI Director James Comey.

But the trajectory would be maddening for any president. He’s gone from Comey telling him he wasn’t suspected of wrongdoing to his personal lawyer being raided. He’s gone from his deputy attorney general blessing a counterintelligence investigation into Russian meddling to blessing FBI agents seizing material related to hush payments to a porn star.

Now it very well may be that every step on that path by Mueller was justified legally, the same as Ken Starr’s trajectory from Whitewater to Monica Lewinsky. But the gravitational pull of such investigations is toward expansion.

By the end, the Starr investigation was a pure partisan fight for power. The same will be true of the Mueller probe, if it isn’t already. This puts the lie to the idea that such investigations can ever be truly above politics.

It’s possible to imagine a different scenario over the past year. Congress could have created an independent commission to investigate Russia. Career prosecutors could have handled the Paul Manafort investigation. The Southern District could have, on its own, taken up the Daniels matter. And if Congress didn’t act or the Trump Justice Department quashed legitimate investigations, Democrats could have used it to build their case for the midterms — and for wielding the subpoena and impeachment power.

This effort wouldn’t have been as centralized and fearsome as Mueller’s operation, but it would have been more politically accountable and open.

All this is moot now, of course, but both sides should eventually consider whether they want another Robert Mueller.

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