YOU'D think that 22 years after the Watergate break-in we would have run out of Watergate mysteries. But a few remain. One of them surfaced in the recently published diaries of H.R. Haldeman, the chief of staff whom President Nixon jettisoned while trying to save his own skin.
This mystery has to do with how, in 1973, Nixon tried to pressure former President Johnson into using his influence with congressional Democrats to call off the Watergate probe by threatening to reveal some Johnson secrets, and how Johnson threatened to retaliate by revealing some dark Nixon secret.
In conversations with Mr. Haldeman, Nixon said he wasn't the first president to eavesdrop on his opponents, that Johnson had done it against the Nixon campaign in 1968.
In 1968, to help Vice President Hubert Humphrey, the Democratic nominee, Johnson was trying to negotiate a settlement of the war that South Vietnam would accept. That would have been the original ``October Surprise.'' Johnson suspected that Nixon's aides were pressing South Vietnamese President Nguyen Van Thieu to refuse a deal and wait for a better one from Nixon.
Democrats have conceded that a wiretap was put on the Watergate apartment (it would have to be Watergate) of Anna Chennault, the so-called ``Dragon Lady.'' She was a Nixon supporter and had high-level contacts in Saigon, which she was suspected of using to stiffen the regime against a preelection settlement.
Nixon kept asking Haldeman to get confirmation from the Federal Bureau of Investigation that the Nixon campaign had also been bugged, but the bureau would provide no such evidence. At one point, Nixon said that anyone in the FBI withholding from him information about Johnson wiretaps would be fired. Haldeman records that Cartha D. (``Deke'') DeLoach, who was FBI director J. Edgar Hoover's top assistant, offered to bring in the file on the Anna Chennault wiretap, but insisted that the FBI had turned down a demand to bug Nixon campaign planes.
Recently, Mr. DeLoach, now retired, told me that Haldeman was inaccurate - that the bugging of Nixon campaign planes had never been requested. What did happen in 1968, he said, was that Attorney General Ramsey Clark made a written request for a check of a call made from the Albuquerque airport on a Nixon campaign stop, possibly to the State Department or others involved with Vietnam negotiations. Actually, DeLoach said, it was not Nixon, but vice-presidential candidate Spiro Agnew who had been in Albuquerque on that day. That did not prevent Nixon from having Haldeman send word to Johnson that if the Watergate investigation was not called off, the Johnson wiretaps and some information about Humphrey contributors would be revealed.
Haldeman's diary entry for June 12, 1973, says that ``LBJ got very hot'' and called DeLoach, saying that ``if the Nixon people are going to play with this, he would release ...'' There follow, in parentheses, the words ``Deleted material - national security.''
This is the only such deletion in some 700 pages of the diaries. Historian Stephen Ambrose, who edited the diaries, says the excision was made by the National Security Council during the Carter administration when Haldeman submitted his diaries for security review. Mr. Ambrose says the deletion is ``the most tantalizing item'' in the entire book.
Today DeLoach says that he is still not free to say what Johnson was threatening to reveal. The deleted passage, he says, contained information classified at a level above top secret. That usually means that disclosure might compromise ``sources and methods'' - espionage agents or electronic eavesdropping techniques.
So what was it Johnson had - or thought he had - on Nixon?
On a guess, it was evidence obtained by wiretaps or an agent in Saigon that, in 1968, the Nixon campaign persuaded the South Vietnamese government to block a peace agreement that Johnson was strenuously trying to negotiate.
Peace before the 1968 election would have powerfully aided Vice President Hubert Humphrey, who was closing in on Nixon in the polls.
But, then, why didn't Johnson announce then that the Nixon campaign was maneuvering to prolong the war? Presumably because he could not afford to reveal how he knew.
And that remains one of the enduring mysteries from old Watergate days. The Opinion/Essay Page welcomes manuscripts. Authors of articles we accept will be notified by telephone. Authors of articles not accepted will be notified by postcard. Send manuscripts by mail to Opinions/Essays, One Norway Street, Boston, MA 02115, by fax to 617 -450-2317, or by Internet E-mail to OPED@RACHEL.CSPS.COM.