Thompson, now preparing a bid for the 2008 GOP presidential nomination, won fame in 1973 for asking a committee witness the bombshell question that revealed Nixon had installed hidden listening devices and taping equipment in the Oval Office.

Those tapes show Thompson played a behind-the-scenes role that was very different from his public image three decades ago. He comes across as a partisan willing to cooperate with the Nixon White House's effort to discredit the committee's star witness.

It was Thompson who tipped off the White House that the Senate committee knew about the tapes. They eventually cinched Nixon's downfall in the scandal resulting from the break-in at Democratic headquarters in the Watergate complex in Washington and the subsequent White House cover-up.

Thompson, then 30, was appointed counsel by his political mentor, Tennessee Sen. Howard Baker, the top Republican on the Senate investigative committee. Thompson had been an assistant U.S. attorney in Nashville, Tenn., and had managed Baker's re-election campaign. Thompson later was a senator himself.

Nixon was disappointed with the selection of Thompson, whom he called "dumb as hell." The president did not think Thompson was skilled enough to interrogate unfriendly witnesses and would be outsmarted by the committee's Democratic counsel.

This assessment comes from audio tapes of White House conversations recently reviewed by The Associated Press at the National Archives in College Park, Md., and transcripts of those discussions that are published in "Abuse of Power: The New Watergate Tapes," by historian Stanley Kutler.

"Oh s---, that kid," Nixon said when told by his chief of staff, H.R. Haldeman, of Thompson's appointment on Feb. 22, 1973.

"Well, we're stuck with him," Haldeman said.

In a meeting later that day in the Old Executive Office Building, Baker assured Nixon that Thompson was up to the task. "He's tough. He's six feet five inches, a big mean fella," the senator told Nixon.

Publicly, Baker and Thompson presented themselves as dedicated to uncovering the truth. But Baker had secret meetings and conversations with Nixon and his top aides, while Thompson worked cooperatively with the White House and accepted coaching from Nixon's lawyer, J. Fred Buzhardt, the tapes and transcripts show.

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Nixon expressed concern that Thompson was not "very smart."

"Not extremely so," Buzhardt agreed.

"But he's friendly," Nixon said.

"But he's friendly," Buzhardt agreed. "We are hoping, though, to work with Thompson and prepare him, if Dean does appear next week, to do a very thorough cross-examination."

Five days later, Buzhardt reported to Nixon that he had primed Thompson for the Dean cross-examination.

"I found Thompson most cooperative, feeling more Republican every day," Buzhardt said. "Uh, perfectly prepared to assist in really doing a cross-examination."

Later in the same conversation, Buzhardt said Thompson was "willing to go, you know, pretty much the distance now. And he said he realized his responsibility was going to have be as a Republican increasingly."

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."