WASHINGTON — President Bush's aides threw up a wall yesterday when questioned about revelations that White House Deputy Chief of Staff Karl Rove had discussed CIA official Valerie Plame with a reporter despite past White House assertions that he was not involved in her unmasking.

Engulfed by questions at two combative briefings, White House press secretary Scott McClellan cited the continuing criminal investigation over the leak to say that he would not discuss conversations Rove had had with a reporter about Plame before her name was published, or say whether Bush's pledge to fire anyone involved in leaking classified information still stands.

"No one wants to get to the bottom of it more than the president of the United States," McClellan said. "And I think the way to be most helpful is to not get into commenting on it while it is an ongoing investigation."

Democrats, emboldened by having the White House on the defensive, began a campaign to pressure Rove to give up his security clearances, answer questions before Congress and even resign.

Whether a crime occurred remains the focus of special prosecutor Patrick Fitzgerald, but the latest revelations also put White House credibility at stake, given past statements by the president, McClellan and others.

Over the weekend, Newsweek reported that Time magazine reporter Matthew Cooper, in an internal e-mail from July 2003, cited Rove as saying that administration critic Joseph Wilson, a former ambassador, had gone to Niger on a fact-finding trip involving Iraq's nuclear-weapons programs at the behest of Plame, Wilson's wife. At the same time, according to Cooper's account, Rove also noted that she worked for the CIA on issues of weapons of mass destruction.

The e-mail did not say that Rove identified Plame by name, and Rove has maintained from the beginning that he neither knew her name nor leaked it to anyone.

Rove's conversation with Cooper took place five days after Plame's husband suggested in a New York Times op-ed piece that the Bush administration had manipulated intelligence on weapons of mass destruction to justify the invasion of Iraq. Wilson has since suggested his wife's name was leaked as retaliation.

Columnist Robert Novak first reported Plame's identity July 14, 2003. It remains unclear whether Rove was a source for Novak, who cited "two senior administration officials" in his column.

The law says that for a crime to have occurred, a government official must have deliberately identified a covert agent and must have known that the agent was under cover and that the government was trying to keep the agent's identity secret.

It was the issue of credibility, more than of criminal culpability, that produced the most aggressive questioning at a White House briefing yesterday — but no answers.

Asked about the matter on nine occasions over the past two years, Bush has said he welcomed the investigation, called the name disclosure "a very serious matter," and declared that the sooner investigators "find out the truth, the better, as far as I'm concerned."

"I want to know the truth," Bush told reporters in September 2003 after news of the investigation had burst into headlines. "If anybody has got any information, inside our administration or outside our administration, it would be helpful if they came forward with the information so we can find out whether or not these allegations are true and get on about the business."

In 2003, McClellan said it was "a ridiculous suggestion" that Rove was involved. "I've made it very clear he was not involved, that there's no truth to the suggestion that he was," he said. He also said any culprit in the White House should be fired "at a minimum."

At one point, the press secretary vowed: "The president has set high standards, the highest of standards, for people in his administration. He's made it very clear to people in his administration that he expects them to adhere to the highest standards of conduct. If anyone in this administration was involved in it, they would no longer be in this administration."

Bush replied "yes" when asked in June 2004 if he would fire anyone who leaked the agent's name.

Democrats seized on that statement yesterday, urging Bush to follow through by dismissing Rove and calling for congressional hearings.

"The White House promised if anyone was involved in the Valerie Plame affair, they would no longer be in this administration," said Senate Minority Leader Harry Reid, D-Nev. "I trust they will follow through on this pledge. If these allegations are true, this rises above politics and is about our national security."

McClellan demurred yesterday when asked several times whether Bush stood by his pledge to fire anyone who leaked classified information. "This question is coming up in the context of this ongoing investigation," he said. "Our policy continues to be that we're not going to get into commenting on an ongoing criminal investigation from this podium."

McClellan's previous denials of White House involvement over nearly two years also occurred when the matter was under investigation. But he said yesterday that at some point after Fitzgerald's inquiry began, "those overseeing the investigation ... said that it would be their preference that we not get into discussing it while it is ongoing."

Robert Luskin, Rove's attorney, acknowledged over the weekend that his client had discussed the subject — without naming Plame — with Cooper, one of the two reporters threatened with jail time for not cooperating with Fitzgerald. Cooper avoided jail last week after being granted a waiver by Rove to testify. New York Times reporter Judith Miller was sent to jail and remains there after refusing to testify.

Newsweek printed the contents of Cooper's July 2003 e-mail, in which he recounts to his bureau chief an interview with Rove that is typical of the cryptic exchanges reporters often have with high-level officials on sensitive matters — vague, but enough to help promote or squelch a story.

It came at a time when the White House was engaged in a major damage-control effort after Wilson said there was no basis for saying Iraq was seeking nuclear-weapons material in Africa.

Luskin said again yesterday that there is nothing inconsistent with what Cooper's e-mail said and what Rove has said throughout the inquiry, and he said his client continues to cooperate fully with Fitzgerald.

At a televised briefing yesterday, reporters grilled McClellan repeatedly by quoting his own words back to him. "I'm well aware, like you, of what was previously said," he responded, "and I will be glad to talk about it at the appropriate time. ... There will be a time to talk about this, but now is not the time to talk about it."

Material from The Associated Press and the Chicago Tribune is included in this report.