The gap was disclosed to Judge John J. Sirica of Federal District Court in 1973, and a panel of experts he appointed had determined in 1974 that it was a result of a deliberate erasure.

The erasure remains one of the great mysteries of the Watergate scandal. But Mr. Haldeman, the president's chief of staff, took diary notes about the conversation that occurred just three days after Nixon campaign operatives were arrested after breaking into the headquarters of the Democratic National Committee in the Watergate office building in Washington.

His diary, published in 1994, said Mr. Nixon ''was concerned about what our counterattack is.''

''He felt we had to hit the opposition with their activities,'' Mr. Haldeman wrote, like ''stealing the Pentagon papers.''

''He feels we should be on the attack for diversion, and not just take it lying down,'' he wrote.

The parts of that tape that remained audible did not mention Watergate.

Mr. Carlin appointed a panel of experts to devise experiments to see if the erased words could be recovered. Five individuals and companies participated in the tests. Two blank tapes from the 1972 White House supply were used. Recordings were made on one of the original White House tape recorders and then erased on the very machine that experts told Judge Sirica in 1974 had been used for the erasure. No intelligible words were recovered from either tape.