In a highly unusual move, key Republicans in Congress are seeking to declassify testimony that former White House terrorism adviser Richard Clarke (search) gave in 2002 about the Sept. 11, 2001, terrorist attack, Senate Majority Leader Bill Frist said Friday.

Frist said the intent was to determine whether Clarke lied under oath — either in 2002 or this week — when he appeared before a bipartisan Sept. 11 commission (search) and sharply criticized President Bush's handling of the war on terror.

"Until you have him under oath both times you don't know," Frist said.

One Republican aide, who spoke on condition of anonymity, said the request had come from House Speaker Dennis Hastert (search) and Rep. Porter Goss (search), the chairman of the House intelligence committee.

The request was the latest evidence of a counterattack against Clarke, who has criticized Bush both in a new book and in his appearance before the bipartisan commission on Wednesday.

In his testimony, Clarke said that while the Clinton administration had "no higher priority" than combatting terrorists, Bush made it "an important issue but not an urgent issue" in the eight months between the time he took office and the Sept. 11 attacks.

Clarke also testified that the invasion of Iraq had undermined the war on terror.

The request for declassification applies to Clarke's appearance in July 2002 before a meeting of the intelligence committees of both the House and Senate.

No immediate information was available on how the declassification process works, but one GOP aide said the CIA and perhaps the White House would play a role in determining whether to make the testimony public.

Frist disclosed the effort to declassify Clarke's testimony in remarks on the Senate floor, then talked with reporter. He said he personally didn't know whether there were any discrepancies between Clarke's two appearances.

Without mentioning the congressional Republicans' effort, White House spokesman Scott McClellan continued the administration's criticism of Clarke on Friday.

"With every new assertion he makes, every revision of his past comments, he only further undermines his credibility," McClellan told reporters.

Asked about Bush's personal reaction to the criticism from a former White House aide, McClellan said, "Any time someone takes a serious issue like this and revises history it's disappointing."