In The Ends of Power, H.R. Haldeman (with Joseph DiMona) offers an idea about the identity of the mysterious “Deep Throat,” the Nixon Administration official who secretly leaked Watergate information to reporters Bob Woodward and Carl Bernstein. Bob Woodward would identify FBI official Mark Felt as Deep Throat, but Haldeman and DiMona wrote The Ends of Power prior to this revelation.
Haldeman speculates that Deep Throat was Fred Fielding, an assistant to White House Counsel John Dean. First of all, on the basis of what Woodward has said, Haldeman notes that Deep Throat was in the Executive Branch and had access to the Committee to Re-Elect the President, the FBI, and the Justice Department when Watergate was going on. According to Haldeman, this would only fit Dean or Dean’s associate.
Second, Haldeman believes that identifying Deep Throat as Fielding would account for the erroneous information that Deep Throat leaked, information that “every White House staffer knew was wrong” (page 187). For example, Deep Throat said that Attorney General John Mitchell investigated Watergate and that E. Howard Hunt (one of the people involved in the break-in) was assigned to gather information, and Haldeman believes that this claim was wrong and “comical”. According to Haldeman, John Ehrlichman and John Dean did the investigating, plus why would Hunt be assigned “to investigate his own break-in” (page 188)? Haldeman notes that Dean said that he kept Fielding “out of things” (Dean’s alleged words) when Watergate was going on, and Haldeman believes this would explain how Deep Throat had incomplete or distorted information.
Do these criteria fit Mark Felt? I guess it depends on when Mark Felt worked for the FBI, and whether that overlaps with the time that Deep Throat was feeding information to Woodward and Bernstein. Felt was in the Executive Branch, yet he was not a close adviser within the Nixon Administration, and (assuming that Felt worked for the FBI when Deep Throat was divulging information) that would be consistent with Deep Throat’s incomplete knowledge. As Ann Coulter notes, however, Felt had left the FBI before the story about the 18 and 1/2 minute gap (which Deep Throat supposedly fed Woodward and Bernstein) came out (see here), so she is skeptical about Felt being Deep Throat. At the same time, Felt probably had contacts within the Executive Branch, and perhaps one of them could have given Felt that information.
(UPDATE: This article indicates that Felt was working for the FBI during some of the time that he was leaking information to Woodward, and also that Haldeman himself feared that Felt would leak information.)