Donald Trump shouldn’t fire Robert Mueller, but he 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 1990s, there was a consensus that we weren’t doing that again, certainly not through the independent-counsel statute, which was allowed to lapse. 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. What resulted were endless politically fraught investigations that often exhibited a zeal disproportionate to the alleged crime.

It’s too early to render a verdict on Mueller’s work, not knowing the underlying facts, but he certainly appears to have become a kind of free-floating legal ombudsman.