Defense Secretary Donald H. Rumsfeld suggested yesterday that U.S. forces in Iraq would isolate Baghdad from its Republican Guard defenders, then wait to see if the city's substantial Shiite Muslim population rises up against Iraqi President Saddam Hussein's Sunni-dominated government.

Rumsfeld, Deputy Defense Secretary Paul D. Wolfowitz and Gen. Richard B. Myers, chairman of the Joint Chiefs of Staff, laid out a relatively sanguine picture of the one-week-old war in back-to-back appearances before Senate and House committees.

But Rumsfeld cautioned that the duration of the conflict and its cost are still unknown. He warned lawmakers that "the campaign could well grow more dangerous in the coming days and weeks as the forces close in on Baghdad and begin to deal with the Republican Guard forces north of Tikrit," Hussein's hometown and power center about 125 miles north of the capital.

Rumsfeld and his aides were summoned to Capitol Hill to discuss the Bush administration's initial request for nearly $75 billion to pay for the war and its immediate aftermath. He made it clear that the request would be a down payment on a military effort that could cost considerably more.

Confronting some skeptical questioning, he offered a glimpse of what may be coming next on the battlefield -- a slow, methodical military campaign to bring Baghdad to heel. Noting the difficult fighting that British Marines have encountered in the southern Iraqi city of Basra, Sen. Pete V. Domenici (R-N.M.) asked, "What's going to happen in Baghdad?"

Rumsfeld replied, "What one has to do is first isolate it."

He said as much as half of Baghdad's 5 million citizens are Shiite Muslims, a population that faces peril from the Iraqi government but nevertheless will help allied forces.

"My guess is that what we'll see is the regime death squads in Baghdad doing what they're doing in Basra," he said. "They'll very likely have weapons out. They'll shoot people who try to surrender. They'll shoot people who try to assist. And we will go through a period where we have to deal with that problem. We'll put in strikes as necessary. We'll undoubtedly get assistance from people inside the cities. And we will attack them and subdue them."

"It could take some time," Rumsfeld added.

Rumsfeld's prediction of assistance from the Shiites is something of a switch from two days ago, when British reports indicated that a Shiite insurrection may have begun in Basra. At that time, Rumsfeld said Iraqi civilians in that city should stay in their homes until allied forces were in a position to help an uprising.

Before the war began, Pentagon officials spoke optimistically of a brittle inner circle around Hussein in Baghdad that would crack quickly under a U.S.-led military assault and a popular uprising. Far from backing away from that hope, Rumsfeld and Wolfowitz said the Iraqi people are still cowering under the threat of the Iraqi leadership and its irregular forces.

Rumsfeld, Myers and Wolfowitz portrayed the war as on track and going well. Myers characterized the guerrilla tactics of Saddam's Fedayeen, the militia under the command of the Iraqi president's son Uday, as more "harassment" than threat, and denied reports that military supply lines were stretched and vulnerable. Rumsfeld said there was no evidence of a "humanitarian crisis" in Iraq.

Rumsfeld also repeated his assertion before the war that the military would not have a protracted role in peacekeeping and reconstruction after the war. Instead, that job will be assumed by a broad international coalition and funded largely by allies, Hussein's seized assets and Iraqi oil, he said.

"I don't believe that the United States has the responsibility for reconstruction in a sense," the secretary said. "What we have is a responsibility to get that country on a path [toward] representative government."

Defense Secretary Donald H. Rumsfeld leans to talk with Deputy Secretary of Defense Paul D. Wolfowitz while testifying before the Senate Appropriations Committee.