Since June 17, 1972, the reporters had saved their notes and memos, reviewing them periodically to make lists of unexplored leads. Many items on the lists were the names of CRP and White House people who the reporters thought might have useful information. By May 17, 1973, when the Senate hearings opened, Bernstein and Woodward had gotten lazy. Their nighttime visits were scarcer, and, increasingly, they had begun to rely on a relatively easy access to the Senate committee's staff investigators and attorneys. There was, however, one unchecked entry on both lists - presidential aide Alexander P. Butterfield. Both Deep Throat and Hugh Sloan had mentioned him, and Sloan had said, almost in passing, that he was in charge of "internal security." In January, Woodward had gone by Butterfield's house in a Virginia suburb. No one had come to the door.

In May, Woodward asked a committee staff member if Butterfield had been interviewed.

"No, we're too busy."

Some weeks later, he had asked another staffer if the committee knew why Butterfield's duties in Haldeman's office were defined as "internal security"

The staff member said the committee didn't know, and maybe it would be a good idea to interview Butterfield. He would ask Sam Dash, the committee's chief counsel. Dash put the matter off. The staff member told Woodward he would push Dash again. Dash finally okayed an interview with Butterfield for Friday, July 13, 1973.

On Saturday the 14th, Woodward received a phone call at home from a senior member of the committee's investigative staff. "Congratulations," he said. "We Interviewed Butterfield. He told the whole story."

What whole story?

"Nixon bugged himself."

He told Woodward that only junior staff members had been present at the interview, and that someone had read an excerpt from John Dean's testimony about his April 15 meeting with the President.

"The most interesting thing that happened during the conversation was very near the end," Dean had said. "He (Nixon) got up out of his chair, went behind his chair to the corner of the Executive Office Building office and in a barely audible tone said to me he was probably foolish to have discussed Hunt's clemency with Colson." Dean had thought to himself that the room might be bugged.

Butterfield was a reluctant witness. He said that he knew it was probably the one thing that the President would not want revealed. The interrogators pressed-and out floated a story which would disturb the presidential universe as none other would.

The existence of a tape system which monitored the President's conversations had been known only to the President himself, Haldeman, Larry Higby, Alexander Haig, Butterfield and the several Secret Service agents who maintained it. For the moment, the information was strictly off the record.

The reporters were again concerned about a White House set-up. A taping system could be disclosed, they reasoned, and then the President could serve up doctored or manufactured tapes to exculpate himself and his men. Or, having known the tapes were rolling, the President might have induced Dean - or anyone else - to say incriminating things and then feign ignorance himself. They decided not to pursue the story for the moment.

All Saturday night, the subject gnawed at Woodward. Butterfield had said that even Kissinger and Ehrlichman were unaware of the taping system. The Senate committee and the special prosecutor would certainly try to obtain the tapes, maybe even subpoena them.

At that very moment, President Nixon was in Bethesda Naval Hospital in suburban Maryland, suffering from viral pneumonia. He had awakened in the early hours of the previous morning with a high fever and complaining of severe chest pains. That day he spent in bed. He had one tense conversation with Sam Ervin in regard to the committee's request for all White House papers that might relate to the Senate's investigation. Nixon had refused to turn over the papers, citing executive privilege. When his condition worsened and a chest X ray showed that he had viral pneumonia, the decision was made to move him to the hospital....

Nixon may well have believed that he was on top of the day's events, but during that weekend the president remained completely unaware that his political fate was being seriously undermined by the prospective testimony before the Watergate committee of Alexander Butterfield.

That Nixon remained ignorant of Butterfield's doings during the weekend of July 14 and 15 has been regarded by Watergate historians and journalists as little more than an oddity to be briefly noted. It was much more than that.

After showing Butterfield to the door, Democratic staffers Armstrong and Boyce rushed to find Sam Dash at his office, while Republican counsel Sanders went on a similar mission to locate Fred Thompson. When Armstrong and Boyce came into his office, Sam Dash later wrote in his book Chief Counsel, "they both looked wild-eyed. Scott was sweating and in a state of great excitement. As soon as he had closed the door the words tumbled out of his mouth as he told me about Butterfield's astounding revelation.... We became overwhelmed with the explosive meaning of the existence of such tapes. We now knew there had been a secret, irrefutable 'witness' in the Oval Office each time Dean met with Nixon, and if we could get the tapes we could now do what we had thought would be impossible-establish the truth or falsity of Dean's accusations against the President."

Thompson was at the bar at the Carroll Arms hotel, having a drink with a reporter, when Sanders dragged him outside to a small park, checked to see if they could be overheard, and blurted out the news.

There were two problems with any proposed Butterfield testimony. The first was that he did not want to testify and had suggested that the committee get Higby or Haldeman to testify in public about the taping system. Second, he was scheduled to leave on Tuesday, July 17, for the Soviet Union to help negotiate a new aviation treaty.

Learning this, Dash found Sam Ervin and they agreed that Butterfield should be compelled to testify on Monday, and Ervin authorized Dash to prepare a subpoena for Butterfield.

For his part, Fred Thompson and Assistant Minority Counsel Howard Liebengood met with Howard Baker on Saturday morning. As Thompson later wrote, "Baker thought it inconceivable that Nixon would have taped his conversations if they contained anything incriminating. I agree d.... The more I thought about what had occurred, the more I considered the possibility that Butterfield had been sent to us as part of a strategy: the president was orchestrating the whole affair and had intended that the tapes be discovered." For that reason, the Republicans came to the same conclusion already reached by Dash and Ervin, that Butterfield should give his testimony in public as soon as possible.

Thompson may well have been correct that Butterfield had been sent to the committee as part of a strategy-but if he was, it was not the president's strategy.

That Saturday morning, as Baker met with his aides, Butterfield flew to New Hampshire to dedicate a new air traffic control facility in Nashua County, and he told us he was so unconcerned about his possible testimony to the Senate that he didn't even prepare for an appearance before the Senate.

"I didn't have the slightest clue" that the committee would call him to testify on Monday, he told us. "No, no-why would I ever do that? I didn't give it one goddamn thought. (Meeting with the Senate staffers) was just another session to me. I know for a fact that I never did anticipate being called by the committee. So I never would have written out any statements, or answers or comments or anything like that having to do with me testifying."

The day before Senate Watergate Committee minority counsel Fred Thompson made the inquiry that launched him into the national spotlight - asking an aide to President Nixon whether there was a White House taping system - he telephoned Nixon's lawyer.

Thompson tipped off the White House that the committee knew about the taping system and would be making the information public. In his all-but-forgotten Watergate memoir, "At That Point in Time," Thompson said he acted with "no authority" in divulging the committee's knowledge of the tapes, which provided the evidence that led to Nixon's resignation. It was one of many Thompson leaks to the Nixon team, according to a former investigator for Democrats on the committee, Scott Armstrong , who remains upset at Thompson's actions.

"Thompson was a mole for the White House," Armstrong said in an interview. "Fred was working hammer and tong to defeat the investigation of finding out what happened to authorize Watergate and find out what the role of the president was."

Asked about the matter this week, Thompson -- who is preparing to run for the 2008 Republican presidential nomination - responded via e-mail without addressing the specific charge of being a Nixon mole: "I'm glad all of this has finally caused someone to read my Watergate book, even though it's taken them over thirty years."

The view of Thompson as a Nixon mole is strikingly at odds with the former Tennessee senator's longtime image as an independent-minded prosecutor who helped bring down the president he admired. Indeed, the website of Thompson's presidential exploratory committee boasts that he "gained national attention for leading the line of inquiry that revealed the audio-taping system in the White House Oval Office." It is an image that has been solidified by Thompson's portrayal of a tough-talking prosecutor in the television series "Law and Order."