US President George W Bush says he will not allow his advisers to take part in "show trials" in the escalating row over the firing of federal prosecutors.
George W Bush said he did not want confrontation with Democrats
He has offered to let his staff give evidence, but only in private and not under oath.
The head of the Senate Judiciary Committee has rejected the offer.
Democrats in Congress have accused Attorney General Alberto Gonzales of sacking prosecutors for political reasons, and called for him to quit.
But Mr Bush has backed Mr Gonzales, a long-time confidant, and warned Democrats against seeking a fight over the issue in order "to score political points".
"We will not go along with a partisan fishing expedition aimed at honourable public servants," Mr Bush said.
He said Democrats should accept his proposal for staff including his chief political adviser, Karl Rove, and his former counsel, Harriet Miers, to meet Senators in private, and that he would fight any effort to subpoena them.
Serve at the discretion of the president, with the advice and consent of the Senate
Prosecute criminal cases brought by the government
Prosecute or defend civil cases in which the government is a party
Collect debts owed to the government
Source: US Department of Justice
"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.
Patrick Leahy, chairman of the Senate Judiciary Committee, rejected his argument.
"Testimony should be on the record and under oath. That's the formula for true accountability," he said.
He said Mr Bush's offer "is not constructive and it is not helpful to be telling the Senate how to do our investigation, or to prejudge its outcome".
Earlier the Senate voted to strip the attorney general of the power to appoint US attorneys without Senate approval.
Mr Gonzales is fighting to hold onto his job
Ninety-four senators voted for the bill, while only two voted against.
The BBC's Jonathan Beale in Washington says that shows the anger in Republican as well as Democrat ranks over the way the Bush administration has handled the row.
Mr Gonzales has denied there was any political motivation for the sacking of eight US attorneys last year, saying their performance was below standard.
However, several leading Democrats, including 2008 presidential hopefuls Hillary Clinton, Barack Obama, and John Edwards, have demanded Mr Gonzales' resignation, as have some Republicans.