WASHINGTON - U.S. President George W. Bush warned Democrats Tuesday to accept his offer to have top aides testify about the firings of federal prosecutors only privately and not under oath, or risk a constitutional showdown from which he would not back down.

Democrats' response to his proposal was swift and firm. "Testimony should be on the record and under oath. That's the formula for true accountability," said Patrick Leahy, chairman of the Senate Judiciary Committee.

Bush, in a late-afternoon statement at the White House, said, "We will not go along with a partisan fishing expedition aimed at honourable public servants. ... I proposed a reasonable way to avoid an impasse."

He added: "There's no indication ... that anybody did anything improper."

Bush gave his embattled attorney general, Alberto Gonzales, a boost during an early morning call to his longtime friend and ended the day with a public statement repeating it. "He's got support with me," the president said.

The Senate, meanwhile, voted to strip Gonzales of his authority to fill U.S. attorney vacancies without Senate confirmation. Democrats contend the Justice Department and White House purged eight federal prosecutors, some of whom were leading political corruption investigations, after a change in the Patriot Act gave Gonzales the new authority.

Several Democrats, including presidential hopefuls Hillary Rodham Clinton, Barrack Obama, Joe Biden and John Edwards, have called for Gonzales' ouster or resignation. So have a handful of Republican legislators.

"What happened in this case sends a signal really through intimidation by purge: 'Don't quarrel with us any longer,"' said Senator Sheldon Whitehouse (D-R.I.), a former U.S. attorney who spent much of Monday evening paging through 3,000 documents released by the Justice Department.

Bush said his White House counsel, Fred Fielding, told legislators they could interview presidential counsellor Karl Rove, former White House Counsel Harriet Miers and their deputies, but only on the president's terms: in private, "without the need for an oath" and without a transcript.

The president cast the offer as virtually unprecedented and a reasonable way for Congress to get all the information it needs about the matter.

"If the Democrats truly do want to move forward and find the right information, they ought to accept what I proposed," Bush said. "If scoring political points is the desire, then the rejection of this reasonable proposal will really be evident for the American people to see."

Bush said he would aggressively fight in court any attempt to subpoena White House aides.

"If the staff of a president operates in constant fear of being hauled before congressional committees ... the president would not receive candid advice and the American people would be ill-served," he said. "I'm sorry the situation has gotten to where it's got, but that's Washington, D.C., for you. You know there's a lot of politics in this town."