The lawmaker promised to make the fight against illegal immigration the cornerstone of his 2008 bid, the report said. "The crisis of illegal immigration threatened not only our economy and our security but our very identity," he said in a statement.

"That ends today," said Tancredo, who, with the announcement, joined a crowded field that includes better-known hopefuls such as former New York mayor Rudy Giuliani and Arizona Senator John McCain.

Born in Denver, Colorado, in December 1945, Tancredo has served as a House member since 1999. He supports deporting all illegal immigrants and has criticized a plan by the President George W. Bush, McCain and others to give some illegal immigrants a chance for citizenship.

An estimated 12 million illegal immigrants now live in the United States. A recent USA Today/Gallup Poll shows immigration ranks fourth behind the Iraq war, terrorism and corruption as an issue that Republicans want the president and Congress to address.

President Bush, making a brief statement to reporters in the Rose Garden, struck an optimistic tone that echoed how aides said Snow was feeling. Mr. Bush said he looked forward to the day when Snow returns to the White House.

"His attitide is one that he is not going to let this whip him," Mr. Bush said. "My attitude is that we need to pray for him."

Snow underwent surgery Monday to remove a small growth in his lower abdomen, a procedure he said last Friday was being done "out of an aggressive sense of caution" because he had colon cancer two years ago.

Doctors determined that the growth was cancerous, and found during the surgery, which was exploratory, that his cancer had metastasized, or spread, to his liver, Perino said.

On the floor of the House of Representatives, Congressman Roy Blunt, R-Mo., said, "I've known Tony, we've all known Tony, for a long time and my belief is if anybody has the stamina and the fortitude and the positive nature to deal with this challenge, he has it," Blunt, who has also battled cancer, said.

"We're wishing him well and frankly hope he is back to work soon, because we need him," Blunt said.

The ad's reach -- it has received more than 1.7 million hits on YouTube in just a few days -- and the unmasking of its maker offer a glimpse of the changed media landscape of a nascent presidential campaign that is bitterly competitive 10 months before first voting.

The new popularity of web video allows broad distribution of clever political messages, which are often negative attack ads that may not readily be traced to their creators.

Phillip de Vellis, a strategist with the Washington-based consulting firm Blue State Digital, which hosts the Obama campaign web site, said he made the ad.

"I did it. And I'm proud of it," Mr de Vellis said in a response posted on a website site this week.

Mr de Vellis said he had acted on his own, without the knowledge of the Obama campaign or his employer.

Its creator remained anonymous.

But for political strategists, ad experts, even journalists, the ad presents a series of other fundamental unknowns.

- How will Web content outside the control of campaigns affect voters?

- How should campaigns react to anonymous but highly viewed attacks?

- When is Web content, no matter how provocative, newsworthy?

As the Internet looks more and more like an electronic community, politicians are increasingly devoting resources to their Web sites, planting themselves in electronic gathering places such as Facebook.com and MySpace.com and posting their videos on YouTube.

With some exceptions, however, what draws viewers is content that politicians don't control. A video clip of former Sen. John Edwards combing his hair to the dubbed-in tune of "I Feel Pretty" has drawn more than 150,000 views. A clip of Clinton singing a slightly off-key version of the Star-Spangled Banner has drawn more than 1 million views.

What's more, Internet content does not have to meet the strict reporting standards that television and radio ads must observe. That makes the Web the medium of choice for stealthy tactics by partisans operating outside the campaigns.

For candidates caught in the crosshairs, one way to respond is to brush it off, preferably with humor.

Asked about the Macintosh video on Tuesday, Clinton said: "I'm just happy if it's taking attention away from my singing. My singing was bad enough. I'm just happy that nobody is tuning in to that."

Republicans on the Senate Judiciary Committee say the Bush administration needs to be more straightforward about the White House's role in the dismissals of eight federal prosecutors.

"I've told the attorney general that I think this has been mishandled, that by giving inaccurate information ... at the outset, it's caused a real firestorm, and he better get the facts out fast," said Sen. John Cornyn, R-Texas.

Sen. Patrick Leahy, D-Vt., the chairman of the committee, pledged to get the public testimony of White House officials involved in the case whether they want to testify or not.

On Monday, the Justice Department planned to turn over to Congress documents that could provide more details of the role agency officials _ including Attorney General Alberto Gonzales _ and top White House officials played in planning the prosecutors' dismissals.

The White House was also expected to announce this week whether it will let political strategist Karl Rove, former White House counsel Harriet Miers and other officials testify or will seek to assert executive privilege in preventing their appearance.

Leahy delayed a vote on issuing subpoenas until Thursday as the president's counsel, Fred Fielding, sought to negotiate terms. But on Sunday, Leahy said he had not met Fielding nor was he particularly open to any compromises, such as a private briefing by the administration officials.

Denouncing a conflict entering its fifth year, protesters across the country raised their voices Saturday against U.S. policy in Iraq and marched by the thousands to the Pentagon in the footsteps of an epic demonstration four decades ago against another divisive war.

A counterprotest was staged, too, on a day of dueling signs and sentiments such as "Illegal Combat" and "Peace Through Strength," and songs like "The Battle Hymn of the Republic" and "War (What's It Good For?)."

Thousands crossed the Potomac River from the Lincoln Memorial to rally loudly but peacefully near the Pentagon. "We're here in the shadow of the war machine," said anti-war activist Cindy Sheehan. "It's like being in the shadow of the death star. They take their death and destruction and they export it around the world. We need to shut it down."

Smaller protests were held in other U.S. cities, stretching to Tuesday's four-year anniversary of the Iraq invasion. In Los Angeles, Vietnam veteran Ed Ellis, 59, hoped the demonstrations would be the "tipping point" against a war that has killed more than 3,200 U.S. troops and engulfed Iraq in a deadly cycle of violence.

"It's all moving in our direction, it's happening," he predicted at the Hollywood rally. "The administration, their get-out-of-jail-free card, they don't get one anymore."

Other protests and counter-demonstrations were held in San Francisco, San Diego and Hartford, Conn., where more than 1,000 rallied at the Old State House.