A note of caution is in order in respect of Attorney General Garland’s appointment of John “Jack” Smith as special counsel to prosecute President Trump. Mr. Smith might be well regarded within the close-knit world of prosecutors. The case that defines his career, though, involved a shocking abuse of prosecutorial judgment. The Supreme Court overturned the conviction and tightened the standard for charging bribery.

That case centered on the prosecution for bribery of a former governor of the Old Dominion, Robert “Bob” McDonnell. When the case began, the governor was one of the highest profile Republicans and a potential candidate for national office. The Obama administration, aided by Mr. Smith, who then headed the Justice Department’s public integrity unit, went after him in a cloud of self-righteousness.

The case was touted as evidence of the department’s commitment to “rooting out public corruption.” Mr. McDonnell and his wife faced up to 20 years in prison. The couple, Justice alleged,  used Mr. McDonnell’s “official position to enrich themselves and their family members.” Yet the Supreme Court would eventually  conclude that the couple were being targeted for commonplace type favors and for taking tawdry, but not illegal, gifts.

When the matter made it before the Supreme Court, the Justices expressed astonishment at the weakness of the government’s case against the McDonnells. The attempt to prosecute them for the run-of-the mill give and take of politics “puts at risk behavior that is common,” Justice Stephen G. Breyer, a Democrat, observed. “That is a recipe for giving the Justice Department and prosecutors enormous power over elected officials.”

For a unanimous court, Chief Justice Roberts vacated the McDonnells’ conviction. He noted that if prosecutors prevailed, it would hobble the work of public officials. They “might wonder whether they could respond to even the most commonplace requests for assistance,” he wrote. Citizens who had “legitimate concerns” might “shrink from participating in democratic discourse.” It all came down to prosecutorial judgment.

Mr. Smith had been brought in “to rebuild” Justice’s public integrity unit in 2010 following another disastrous attempt to prosecute a public official — Senator Stevens of Alaska. His conviction had been tossed out due to misconduct by prosecutors, who concealed evidence from defense lawyers. Yet Mr. Smith broadcast his eagerness to put public officials in the dock.

Mr. Smith “emphasized” that under his leadership, Los Angeles Times reported, “the unit dramatically increased the number of cases that went to trial.” A prosecutor in the unit, Edward Loya Jr., told the New York Times in 2014 that he was struck by Mr. Smith’s eagerness “to get us trials,” noting that some in the office “were turned off by how aggressive he was.”

Mr. Smith was unchastened. “Being able to try a case is what gets defense attorneys to take you seriously,” Mr. Smith told the Times. “It’s what gets guilty people to plead guilty.” It’s all too common for innocent persons, too, as these columns pointed out when the Justice Department dropped its charges against General Michael Flynn, who had pleaded guilty to a crime he didn’t commit.

Which brings us back to President Trump, though our concern is not with him, per se. We are outlining here concerns on which we defended Presidents George H.W. Bush and Clinton against special or independent prosecutors. And we get that Attorney General Garland is pledging that the department will “make decisions indisputably guided only by the facts and the law.”

Yet the department’s — and Mr. Smith’s — record is mixed. The temptation of a special counsel to prosecute has proven too much even for veteran lawyers. Mr. Smith’s penchant for using his eagerness “to try a case” to frighten his targets will make it worse. This is to what Justice Antonin Scalia was referring in his immortal warning of independence in a prosecutor. “Frequently,” The Great Scalia wrote, “an issue of this sort will come before the Court clad, so to speak, in sheep’s clothing . . . But this wolf comes as a wolf.”