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."

But the story of his role in the Nixon case helps put in perspective Thompson's recent stance as one of the most outspoken proponents of pardoning I. Lewis "Scooter" Libby, the former chief of staff to Vice President Dick Cheney. Just as Thompson once staunchly defended Nixon, Thompson urged a pardon for Libby, who was convicted in March of obstructing justice in the investigation into who leaked a CIA operative's name.

Thompson declared in a June 6 radio commentary that Libby's conviction was a "shocking injustice . . . created and enabled by federal officials." Bush on Monday commuted Libby's 30-month sentence, stopping short of a pardon.

The intensity of Thompson's remarks about Libby is reminiscent of how he initially felt about Nixon. Few Republicans were stronger believers in Nixon during the early days of Watergate.

Thompson, in his 1975 memoir, wrote that he believed "there would be nothing incriminating" about Nixon on the tapes, a theory he said "proved totally wrong."

"In retrospect it is apparent that I was subconsciously looking for a way to justify my faith in the leader of my country and my party, a man who was undergoing a violent attack from the news media, which I thought had never given him fair treatment in the past," Thompson wrote. "I was looking for a reason to believe that Richard M. Nixon, President of the United States, was not a crook."