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An administration official familiar with the nomination, who asked not to be named because the matter involved personal disclosure issues, said Mr. Horowitz would not require a waiver. The official also said that when a nominee has confidential clients, “the agency counsels the nominee on applicable ethics rules and helps them to set up a screening process to ensure that they are recused from matters involving those clients.

“In this case, [the Justice Department] reviewed his disclosed client list and determined that Mr. Horowitz would be able to implement a screening and recusal process to ensure compliance with the Ethics Pledge, as well as the other applicable ethics regulations, for all of his clients,” the official said.

A former federal prosecutor, Mr. Horowitz served on the U.S. Sentencing Commission and held top Justice Department posts in the George W. Bush and Clinton administrations, including deputy assistant attorney general and chief of staff to the head of the criminal division.

Through a spokesman for the Cadwalader, Wickersham & Taft law firm, where Mr. Horowitz is a partner, he declined to comment. The law firm provided several names of references, including two officials who served during the Bush administration.

Michael Chertoff, former Homeland Security secretary and former judge on the U.S. Court of Appeals for the 3rd Circuit in New Jersey, said Mr. Horowitz is “a good choice.”

“He’s fair, even-tempered and not a guy who is going to buckle under pressure … a model of integrity,” Mr. Chertoff said.

Larry Thompson, a deputy attorney general at the Justice Department from 2001 to 2003, said he, too, had a handful of clients from private practice whose identities he was bound not to disclose when he went to the department.

“I think certainly a nominee with the respect and demonstrated integrity as Michael would recuse himself from those matters,” Mr. Thompson said.

Mr. Horowitz isn’t the only nominee who has declined to divulge some of his clients.

Paul J. Fishman, U.S. attorney for New Jersey, provided the names of 29 clients on disclosure forms he submitted to the U.S. Office of Government Ethics in 2009, but withheld the names of “approximately 37 confidential clients” involved in grand jury or other secret investigations.

Frank Kendall III, principal deputy undersecretary for acquisition, technology and logistics at the Defense Department, declined to name six consulting clients. In the cases of Mr. Fishman and Mr. Kendall, White House officials said they were confident that the appointees would recuse themselves in cases involving former clients.

Mr. Horowitz was nominated for the inspector general’s job to replace Glenn A. Fine, who resigned this year after nearly a decade in the prominent watchdog position. Under Mr. Fine, the office headed high-profile investigations into the firings of U.S. attorneys and raised questions about the FBI’s use of national security letters.

In 2008, Mr. Fine’s office found that as U.S. attorney general, Alberto Gonzales failed to properly secure 18 top-secret documents related to National Security Agency surveillance and detainee interrogation programs.

More recently, Mr. Fine’s office was investigating the Justice Department’s civil rights division to determine whether voting section employees were harassed for participating in specific investigations or prosecutions. The move was made amid concerns within and outside the Justice Department about the dismissal of a complaint brought against the New Black Panther Party after its members disrupted a Philadelphia polling place.