Eric Holder in Ferguson

Attorney General Eric Holder, left, meets with Missouri Highway Patrol Capt. Ron Johnson in Ferguson, Mo., on Wednesday. (J.B. Forbes, McClatchy-Tribune / August 20, 2014)

Though I've known prosecutors to try their cases in front of television cameras, Attorney General Eric Holder's visit to embattled Ferguson, Mo., left me with an especially queasy feeling.

Nearly six years ago, Patrick Fitzgerald, at the time U.S. attorney for the Northern District of Illinois, announced the arrest of Rod Blagojevich, then the governor of Illinois, with a great rhetorical flourish.

Fitzgerald said Blagojevich was in the midst of "a political corruption crime spree" that "would make Lincoln roll over in his grave." That was long before a jury would get a shot at testing those ringing phrases for truth. But that case wasn't surrounded by the high-decibel emotions released by the killing of an unarmed black teenager by a white cop in Ferguson. Illinoisans take political corruption pretty much in stride, considering it regrettable but inevitable. Whatever the verdict in Blagojevich's case, there was little chance protesters would flood Daley Plaza, let alone pelt cops with missiles and insults.

But the killing of Michael Brown occurred against a backdrop of America's ever-widening racial division. Recently I wrote a piece on Watergate, pegged to the 40th anniversary of President Richard Nixon's resignation. For weeks afterward, I got emails demanding that I tell the truth about President Barack Obama's crimes which, my correspondents assured me, dwarfed whatever mistakes Nixon might have made. Yet the vagueness of the allegations against Obama strongly suggested that, in the eyes of those angry critics, his crime was simply being black.

Meanwhile, as the protests and acts of violence in Ferguson demonstrate, many black Americans are convinced that the scales of justice are tipped against them. That was the theme of the attorney general's trip to Ferguson, where he met with the family of the slain man.

"I understand that mistrust," Holder said. "I am the attorney general of the United States. But I am also a black man."

The purpose of Holder's visit was to assure the angry voices, not just in Ferguson, but across America, that justice will be done. Paradoxically, his words put that goal in jeopardy. Supporters of the Brown family have called for the local prosecutor to be replaced, arguing that he views the case through the prism of his own experience: His father, a police officer, was killed while responding to an incident involving a black suspect. That's a reasonable assumption. But by the same logic, shouldn't Holder have stayed away from the case? He has made a special point of viewing it through the prism of his experiences.

The local prosecutor may or may not be taken off the case. Holder won't be in the courtroom, but his words will echo. There's a courtroom no-no called vouching — meaning that a prosecutor can't, by word or body language, tell a jury: "Trust that witness because I put him on the stand, and I represent the government."

When that happens, a defense attorney objects and the judge tells the jurors to disregard what they heard. But that defense against tainted testimony won't be available in this case — posing the question of whether the police officer who fired the fatal shots would get a fair trial if he is charged. Would jurors be listening to the witnesses, or remembering Holder's words?

That is not to say that Holder's decision to insert himself in this case — going so far as to visit the locale of the tragedy — was badly taken. With protesters and peace officers engaged in confrontations, it may well have behooved the nation's highest law enforcement officer to try to defuse the tension.

But in politics, no less than physics, for every action there is an equal and opposite reaction. Let's pray that the kickback from this one doesn't do further damage to the already fragile relations between black and white Americans.

Ron Grossman is a Tribune reporter and former history professor.