Sen. John McCain, the leading critic of the recent spate of national security leaks, rejected Attorney General Eric H. Holder Jr.’s appointment on Friday of two U.S. attorneys to investigate the breaches, arguing that only an outside counsel would be independent enough for such a task.

Mr. McCain, an Arizona Republican who lost the 2008 presidential race to President Obama, as well as Sen. Lindsey Graham, South Carolina Republican, issued a joint statement late Friday night, arguing that Mr. Holder’s decision to tap the two U.S. attorneys doesn’t go far enough.

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“These breaches of national security have compromised operations, strained relationships with allies, and put lives at risk. It is imperative that an independent investigation be conducted where the results could be accepted with a high degree of confidence and without a hint of political considerations,” they said. “We are confident the two U.S. attorneys hand-picked by Attorney General Holder are fine men. However, if there was ever a situation where we needed an outside special counsel that would enjoy bipartisan acceptance and widespread public trust, it is now.”

Since an article about U.S. cyberattacks on Iranian computers appeared in the New York Times last week, Mr. McCain has railed against the leaks, blaming them on administration officials trying to bolster Mr. Obama’s foreign policy credentials ahead of the November election.

Other prominent senators on both sides of the aisle, including Democratic Sens. John F. Kerry of Massachusetts, who chairs the Foreign Affairs Committee, and Dianne Feinstein of California, who chairs the Intelligence Committee, have condemned the leaks and pledged to lead their own investigations to figure out where they are coming from.

“This investigation involves some of the most serious breaches of national security in recent memory and any investigation must be done in a manner free and clear of political considerations,” Mr. McCain and Mr. Graham said. “The recent decision of the attorney general falls far short of what is needed and is not an adequate substitute for an outside special counsel.”

The two Republican senators suggested Bob Bennett, a partner at the law firm Hogan Lovells who is best known for representing former President Bill Clinton during the sex scandal involving Monica Lewinsky, as a suitable candidate to lead a nonpartisan probe.

“There are many capable attorneys who have earned respect from the public and both parties. We strongly believe a special counsel should be appointed outside Justice Department control and influence. Someone in the mold of a Bob Bennett, who is extremely competent and enjoys bipartisan respect,” they said.