With Tropical Storm Rita bearing down on the Gulf of Mexico and growing political pressure from federal leaders, Mayor C. Ray Nagin said Monday that New Orleans residents could not return home after all and that any people already in the city should evacuate.

Nagin had been allowing business owners to return over the weekend, and on Monday residents of one dry neighborhood were to return to their homes. But Nagin reversed himself and ordered another mandatory evacuation, to begin Wednesday, just hours after President Bush questioned whether the city was safe enough for people to return.

"We are suspending all reentry into the city of New Orleans as of this moment," Nagin said. The mayor said he backed away from his earlier decision because of fresh fears about Rita, which forecasters said could become a hurricane by Tuesday.

"If we are off, I'd rather err on the side of conservatism to make sure we have everyone out," Nagin said.

The city's levees, overwhelmed by Hurricane Katrina, "are still in very weak condition" and many of the pumps used to push the mucky floodwaters back into Lake Pontchartrain are not yet operating, Nagin said. If Rita were to dump nine inches of rain on New Orleans, the result would be "three to four feet of flooding in most parts of the city," he said.

Asked if anything could be done to buttress the levee system before Rita were to strike, Nagin replied: "Just tell people to run."

Current weather projections indicate that Rita, which was threatening Key West, Fla., Monday night, could roar across the Gulf and strike the lower portions of Louisiana by the weekend. If, as some suggest, New Orleans sits on the "eastern side of the storm, we take the brunt of it," Nagin said.

As New Orleans residents faced the grim prospect of an even slower recovery, state officials put the still-rising Katrina death toll at 736 in Louisiana and the overall toll at 973.

Bush administration officials and Nagin have sparred publicly and in private in recent days over the mayor's push to demonstrate that New Orleans will be back in business soon. Last Thursday, he laid out a plan to permit up to 182,000 people to return over the course of 10 days.

Vice Adm. Thad W. Allen, the Coast Guard chief of staff tapped by Bush to lead the federal response here, said that move was "extremely problematic." Allen said it was dangerous to invite tens of thousands of people into a city with little clean water, a severely compromised sewer system, a manual 911 emergency call system and few hospitals or traffic lights.

Nagin, interviewed over the weekend in Dallas by Fox News, questioned Allen's credentials: "Since I have been away a day or two, maybe he's the new crowned federal mayor of New Orleans."

That prompted Bush to reinforce Allen's message, telling reporters he was taking the unusual step of commenting publicly to be certain the mayor got the message.

"We have made our position loud and clear," the president said Monday.

Although he defended his initial plan to reopen New Orleans, Nagin said the new decision to evacuate was based solely on long-range projections that show Rita has the potential to bring high winds and downpours to a city ill-equipped to handle another natural disaster.

"This is not a diversion," he said from his new command center in a downtown hotel. "This is a real threat."

Initially criticized for failing to move many of the city's impoverished black residents out on the weekend prior to Katrina, Nagin promised a more aggressive approach to the evacuation set to begin Wednesday.

"I don't play around with hurricanes," Nagin said. "I've seen Katrina."

Although Nagin said he would turn to active-duty soldiers and the National Guard to help the city's depleted police department enforce the new evacuation order, a Pentagon official said that is not the role of the military.

"They have not asked us to help them take anyone out of the city at this time," said Brig. Gen. Mark A. Graham, deputy commanding general of Fifth U.S. Army, which oversees the 82nd Airborne Division and other active-duty Army forces in the region. "We don't forcibly evacuate anyone. That's a law enforcement job."

The Department of Defense was also evaluating whether to send out to sea vessels such as the warship USS Iwo Jima, which is tied up at a Mississippi River dock in downtown New Orleans and is serving as command center for the military in the region.

"I'm certain they are contemplating that" because ships "have a better chance of riding out the storm" at sea than if they are tied up to moorings, said Maj. John Thomas, a spokesman for the federal recovery effort in Baton Rouge, La.

Many emergency responders who have helped rescue survivors and locate the dead here were pondering their next move, weighing whether they ought to flee before Rita's arrival or hunker down.

Alexandria, La., Police Capt. John Henderson, who is helping run the city's command center, said aides were drawing up evacuation plans that would enable out-of-town relief workers to move to high ground if necessary later in the week.

Throughout the day, there were pockets of activity in Algiers, the neighborhood across the Mississippi River where residents were officially permitted to return. Even before Nagin's announcement, most residents said they had no intention of staying.

"I just came to clean up, salvage as much as I can," said Ramsey Washington, who drove back from his temporary home in Houston. "It's not livable. They got no jobs. No banks are open."

Sonia Badon, 35, has already rented an apartment in Baton Rouge; her employer has relocated his computer company there, too. On Monday, she and her boss and two friends were nailing a tarp onto the roof of her Halsey Avenue home.

Although her best friend has moved to Nashville in search of a job, Badon said she will return to her native New Orleans. But for now, she said with sweat rolling down her face, "It's too soon to come back."

Staff writer Ann Scott Tyson in Washington contributed to this report.

Mayor C. Ray Nagin said Rita's rainfall could overwhelm the damaged levees in New Orleans.