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President Trump treated the appointment of Robert Mueller as a special counsel investigating Russia's role in the election with his customary petulance. He began the day Thursday with a self-pitying tweet claiming that he's the victim of the "single greatest witch hunt of a politician in American history!" Later, he told network anchors that the appointment "hurts our country terribly."

Actually, the appointment is a huge victory for the rule of law. Mueller, who had preceded James Comey as FBI director, is a public servant of the highest order: a decorated Vietnam War veteran, career prosecutor and senior Justice Department official under presidents of both parties.

Mueller, 72, comes to his new position with an immense amount of credibility and should be able to get to the bottom of connections between Russia and the Trump campaign, and whether there was an effort to obstruct the investigation.

But Mueller's appointment is not enough. Comey's replacement to head the FBI still has to be made. That appointment is vital for a number of reasons, not the least of which is that FBI agents will make up a good bit of Mueller's investigative team.

Trump's list of potential nominees has been troublingly long on politicians and short on career prosecutors and law enforcement officers. As of Thursday, the front-runner was said to be former Connecticut senator Joe Lieberman, a Democrat-turned-independent with a history of crossing party lines, including endorsing John McCain over Barack Obama for president in 2008.

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Other politicians said to be in the running were former governor Frank Keating, R-Okla., and former congressman Mike Rogers, R-Mich. (Another leading contender, Sen. John Cornyn, R-Texas, took his name out of consideration.)

All are solid people, but none would make an appropriate FBI director.

Lieberman, in particular, should raise eyebrows. At 75, he is hardly of an age one would expect to start on what is normally a 10-year term. He has no federal law enforcement experience. He is a former state attorney general, but those jobs are much more political than their federal counterparts and don't generally involve a lot of criminal prosecution.

There is a reason for the lack of politics on the résumé of past FBI directors. The position is one that needs be free of party allegiances and even of an appearance of being beholden to presidents.

Since its creation in the early 20th century, the FBI and its precursor agency, the U.S. Bureau of Investigation, have had 19 permanent and acting directors. Of these, none could be called a career politician; only two of them, both acting directors, had short tenures in elective office early in their careers.

Now is not the time to change that.

A more appropriate choice would be a career prosecutor, a high-level law enforcement officer, a judge, or perhaps a senior intelligence agency administrator. In other words, the best pick would be a lot like a younger version of Robert Mueller.

USA TODAY's editorial opinions are decided by its Editorial Board, separate from the news staff. Most editorials are coupled with an opposing view — a unique USA TODAY feature.

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