Although “Deep Throat” became for 33 years the most famous mystery source in investigative reporting, it is worth recalling that his momentous Watergate disclosures had little immediate public impact. Richard Nixon gained re-election in 1972 by a landslide despite the torrent of coverage of his campaign's misdeeds in The Washington Post stemming from the fabulous insider source of Bob Woodward, then a cub reporter.

But if America ignored it at first, what Deep Throat - Mark Felt, who died on Thursday - had done, as Woodward later wrote, was to set out a “road map”. It was followed by judges, prosecutors, and congressmen. Over the next two years it proceeded from the implausible to the unthinkable - the first-ever removal of a president from office.

Ever since, Watergate has been the mother of all scandals, seen as the acme of the best reporting. Certainly it changed dramatically how reporters and politicians viewed each other. But in 1972 not many of the press, including me, took it that seriously.

Obviously the break-in at Democratic Party headquarters in the Watergate building by Nixon campaign employees stank as a dirty trick. But the cover-up worked. The rest of us didn't have Felt as a source. We watched with amazement the audacity of Woodward and Carl Bernstein. But the weakness of the Democratic candidate, George McGovern, meant that voters stuck with the devil they knew. Of the TV networks, only CBS on the eve of the election gave the Watergate scandal a close look, to little effect.

Times Archive, 1974: Backroom boys of Watergate

Bernstein and Woodward identify their main source as 'Deep Throat', who prosecutors believe was Mark Felt Jr, a deputy associate director of the FBI

• Guessing the identity of 'Deep Throat', 1976

• Archive Topic: Watergate and the Fall of Nixon

Even The Washington Post went quiet after the election. It was not until after the first trial of those arrested at Watergate, and the squealing to the judge of one of those convicted, that the affair exploded. This is the period in 1973 that most people remember: the televised Senate committee hearings; the endless breathtaking developments; high-level resignations; sackings; the critical revelation that Nixon had recorded almost all his of conversations; the court battle for the tapes. Finally the impeachment hearings and Nixon's resignation. That was the public side.

But thanks largely to those tapes we know that the inside story was frantic from the start. The reason that Nixon would not risk making a quick clean breast of the break-in, condemning those involved, and claiming they had exceeded orders - which he probably would have got away with - was simple. Simple with hindsight, that is. It was that the President knew the same gang had carried out a break-in on White House orders the year before. Their target was the psychiatric files of Daniel Ellsberg, a former official who leaked the Vietnam official archive known as the “Pentagon Papers”.

This had infuriated Nixon, especially as he was engaged in secret diplomacy with China and North Vietnam. The White House was desperate to discredit Ellsberg - and used the break-in team because it could no longer get the FBI to do the dirty work. There was a whole chain of illegalities that would risk unwinding. Had that happened Nixon would probably have lost the election.

We know from the tapes that Nixon and his chief of staff, H.R. Haldeman, almost immediately after the Watergate break-in began scheming to have the CIA warn the FBI off from investigating on “national security” grounds. Although the scheme failed, the court-enforced disclosure of this tape more than two years later led immediately to Nixon's resignation.

Since the No2 at the FBI at the time of Watergate was Felt, we can now place him at the very centre of the conspiracy, exquisitely placed to exploit what he knew.

As it happened he had met a young naval lieutenant in a White House waiting room in 1970 and developed a paternal kind of relation with him. By 1971, the young man, Woodward, was a reporter and had already in mid-1971 received a tip from Felt that Vice-President Agnew was taking bribes - but coming from so junior a journalist, it was not followed up. Two years later Agnew had to resign over the issue.

Woodward got the weekend assignment to cover the first low-level court hearing of those arrested at the Watergate building. By Monday he was telephoning Felt and getting guidance, and the rest is history. Their spy-like rendezvous in an underground garage were first related in the “Woodstein” book, then the film All the President's Men, although the identifying details had to await publication in 2005 of Woodward's book The Secret Man.

Felt, right until his 2005 “outing” by his lawyer and daughter in Vanity Fair, denied that he was “Deep Throat”. Only six people knew his identity, the two reporters, one of their wives, two editors, and a Department of Justice lawyer who rumbled him in 1976.

We can only speculate as to why such a top intelligence operative should turn whistle-blower. Certainly he was embittered at being passed over, twice, for the top job that he coveted. He despised Nixon's attempts to gain political control of FBI functions - but, as Woodward reveals, Felt was not averse to taking the law into his own hands. It is a supreme irony that when Felt was put on trial in connection with FBI break-ins he had authorised against Weather Underground, the domestic terrorist group - a witness in his defence was former President Nixon.

It is tragic that, when Woodward visited him in 2000, Felt could no longer remember much, and thus could not, alas, make his million out of a Watergate book, as his daughter evidently hoped. So if his unmasking was an anti-climax, I think the best memorial to him is Hal Holbrook's portrayal as the rasping-voiced shadow talking to Robert Redford in the film All the President's Men.

Obituary, page 67

Fred Emery was chief Washington correspondent of The Times 1970-77 and is author of Watergate: The Corruption and Fall of Richard Nixon

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