WASHINGTON - President Bush invoked executive privilege Monday to deny requests by Congress for testimony from two former aides about the firings of federal prosecutors.

The White House, however, did offer again to make former counsel Harriet Miers and one-time political director Sara Taylor available for private, off-the-record interviews.

In a letter to the heads of the House and Senate Judiciary panels, White House counsel Fred Fielding insisted that Bush was acting in good faith and refused lawmakers’ demand that the president explain the basis for invoking the privilege.

“You may be assured that the president’s assertion here comports with prior practices in similar contexts, and that it has been appropriately documented,” the letter said.

House Judiciary Committee Chairman John Conyers offered a stinging response. “Contrary what the White House may believe, it is the Congress and the courts that will decide whether an invocation of executive privilege is valid, not the White House unilaterally,” the Michigan Democrat said in a statement.

The exchange Monday was the latest step in a slow-motion legal waltz between the White House and lawmakers toward eventual contempt-of-Congress citations. If neither side yields, the matter could land in federal court.