For the first time since taking office, President Bush faces the possibility that large numbers of American troops may die on his watch, and his aides are scrambling to prepare the public for a toll that could rapidly erode support for the war.

White House officials said that as one way of confronting setbacks or persistent bad news, Bush will make frequent public appearances that emphasize progress -- however incremental -- toward the ultimate goal of disarming Saddam Hussein and liberating Iraq.

"It is a war, and there will be sacrifices made by our men and women who wear the uniform," communications director Dan Bartlett said. "The president has explained to the American people that these sacrifices will make our country safer by saving countless lives from catastrophic attack."

The administration has adjusted its message to include much blunter assessments of the risks ahead. Defense Secretary Donald H. Rumsfeld warned yesterday that the campaign "could well grow more dangerous in the coming days and weeks as coalition forces close on Baghdad and the regime is faced with its certain death."

That is a significant shift in emphasis from the run-up to the war, when Bush was trying to sell his policy. Vice President Cheney said on NBC's "Meet the Press," only days before the war started, that he guessed that significant elements of Hussein's elite Republican Guard would be likely "to want to avoid conflict with U.S. forces and are likely to step aside."

Bush began preparing the public for sacrifice the next day, saying in an address to the nation that Americans "understand the costs of conflict, because we have paid them in the past." Democrats say that was too late, leaving Americans unprepared for the possible cost in lives, dollars and time.

The toll could rise much higher than the 150 allied deaths in the Persian Gulf War of 1991. Retired Army Gen. Barry R. McCaffrey, a mechanized-infantry commander in that war, told the BBC that the allies' death toll in Iraq could reach 3,000.

Casualty counts have caused two recent administrations to retreat from overseas missions. President Bill Clinton withdrew troops from Somalia after the 1993 slaughter of 18 commandos during fighting in Mogadishu. President Ronald Reagan withdrew peacekeepers from the civil war in Lebanon after 241 Marines were killed in the bombing of their barracks in Beirut in 1983.

White House officials said they intend to stick with the military strategy, and already are taking steps to mitigate the political cost of setbacks. Bush has given less public attention to the early casualties in Iraq than he did to the first casualties in Afghanistan.

This week, Bush said he prayed for "those in the coalition forces who lost their lives," and "for their families." A senior White House official said it is Bush's practice to send a hand-signed note to the families of military casualties of the war, "in which he would typically express that the nation mourns those who lost their lives in service to our country on the field of battle."

The Pentagon said it was not part of a strategy, but several of the identifications of casualties in Iraq have been released a day or more after the next of kin said they were notified.

Opinion is mixed about the public's tolerance for casualties. Eric V. Larson, a Rand policy analyst who specializes in war and public opinion, said he believes the public can endure relatively high casualties in Iraq because the replacement of Hussein has long been considered unfinished business by a strong majority of Americans, and because most people see weapons of mass destruction as a serious threat and view Iraq as a vital interest to the United States.

A variety of other pollsters and political scientists said support for Bush and the war could ebb rapidly if casualties mount, because the wartime rally in public opinion has been fragile and has excluded most Democrats. "If you have as much doubt going in as they did and have so few countries on board, and throw in an unanticipatedly high level of casualties," said David Kennedy, a presidential scholar at Stanford University, "that is a formula for real political disaster."

A Washington Post-ABC News poll conducted Sunday night found that 54 percent of respondents believed the allies would sustain "significant" casualties in the war, up from 37 percent on Thursday.

By last night, 21 U.S. troops had been killed in the war in Iraq, and 14 were captured or missing.

Staff writers Amy Goldstein and Richard Morin contributed to this report.