President Obama on Friday nominated James Comey to run the FBI, and the former prosecutor and deputy attorney general is already garnering media effusions reserved for any Republican who fell out publicly with the Bush Administration. Forgive us if we don't join this Beltway beatification.

Any potential FBI director deserves scrutiny, since the position has so much power and is susceptible to ruinous misjudgments and abuse. That goes double with Mr. Comey, a nominee who seems to think the job of the federal bureaucracy is to oversee elected officials, not the other way around, and who had his own hand in some of the worst prosecutorial excesses of the last decade.

***

The list includes his overzealous pursuit, as U.S. Attorney for New York's Southern District, of banker Frank Quattrone amid the post-Enron political frenzy of 2003. Mr. Comey never did indict Mr. Quattrone on banking-related charges, but charged him instead with obstruction of justice and witness tampering based essentially on a single ambiguous email.

Mr. Comey's first trial against Mr. Quattrone ended in a hung jury; he won a conviction on a retrial but that conviction was overturned on appeal in 2006. This May, the Quattrone Center for the Fair Administration of Justice was launched at the University of Pennsylvania thanks to a $15 million gift from the banker, perhaps with Mr. Comey partly in mind.

There is also Mr. Comey's 2004 role as deputy attorney general in the Aipac case, in which the FBI sought to use bogus "secret" information to entrap two lobbyists for the pro-Israel group and then prosecuted them under the 1917 Espionage Act. The Justice Department dropped that case in 2009 after it fell apart in court—but not before wrecking the lives of the two lobbyists, Steven Rosen and Keith Weissman.