But Mr. Snow said, “If they issue subpoenas, the offer is withdrawn, because it means that they will not have responded to the offer. They will have rejected the offer.”

“The moment subpoenas are issued, it means that they’ve rejected the offer,” Mr. Snow emphasized.

Responding defiantly on Tuesday, Mr. Bush said he would resist any effort to put his top aides under “the klieg lights” in “show trials” on Capitol Hill, and he reiterated his support for Attorney General Alberto R. Gonzales, whose backing among Republicans on Capitol Hill ebbed further.

“We will not go along with a partisan fishing expedition aimed at honorable public servants,” the president told reporters in a brief and hastily convened appearance in the Diplomatic Reception Room of the White House.

The pointed exchange was set off by a Democratic inquiry into whether the White House let politics interfere with law enforcement by dismissing eight of the nation’s 93 United States attorneys. The dismissals and the way the Justice Department informed Congress about them have created an uproar in both parties, as Republicans and Democrats on Capitol Hill have demanded explanations.

Tuesday’s confrontation was the sharpest yet between the Bush White House and the new Democratic majority in Congress on a matter of oversight, and it set the stage for a legal showdown over executive privilege. The White House vowed to fight any subpoenas in court.

The fallout from the dismissals continued to ripple across the capital.

In the Senate, lawmakers responded to the furor over the firings on Tuesday by voting overwhelmingly to revoke the authority they had granted the administration last year under the USA Patriot Act to install federal prosecutors indefinitely without Senate confirmation.

Lawmakers also spent the day poring over 3,000 pages of newly released e-mail messages that provided a glimpse inside the Justice Department as officials planned the dismissals and then reacted to the issue as it ignited into a political crisis.