Susan Threadgill writes in the July/August issue of The Washington Monthly magazine:

"H.R. 'Bob' Haldeman's recent diaries are chock full of seamy details about the presidency of Richard M. Nixon.

"One of the seamiest is the hitherto unknown story of how Nixon tried to blackmail Lyndon Johnson into using his influence with congressional Democrats to help halt the Watergate investigation.

"Nixon had threatened to reveal that Johnson had bugged him during the 1968 campaign. But Johnson let Nixon know that if Nixon did that, LBJ would release what the diaries then refer to as 'deleted material -- national security.' "

Ms. Threadgill offers her guess that the deleted material "was probably evidence that the Nixon forces had conspired with South Vietnamese leaders to sabotage October 1968 peace negotiations that, if successful, would have helped the Democrats.

"The irony is that if LBJ had released his evidence before the 1968 election, Nixon might have been defeated. But Johnson was said to be so angry at Hubert Humphrey for not supporting LBJ's policy in Vietnam that LBJ refused to make the Nixon sabotage public."