Criticized for weeks for failing to react aggressively enough to Hurricane Katrina (search), President Bush on Monday put some muscle behind concerns that a new tropical storm could again deluge New Orleans (search) and the people being allowed to return to the ruined city.

It worked. Six hours after Bush questioned New Orleans Mayor Ray Nagin's (search) plan to bring as many as a third of the city's population back over the next week, Nagin reversed course and said he was suspending it.

Appearing at the White House with his homeland security council, Bush said he had "deep concern" about the possibility that Tropical Storm Rita (search), which was headed toward the Florida Keys, could move into the Gulf of Mexico and impact New Orleans.

"If it were to rain a lot, there is concern from the Army Corps of Engineers (search) that the levees might break," Bush said. Among the many other concerns are a lack of drinkable water and fears about contamination levels in the remaining floodwaters and the muck left behind in drained areas.

As Bush spoke, residents had begun trickling back into the city.

Besides the president's public words, White House chief of staff Andrew Card (search) pressed the point on the phone with Nagin. And Coast Guard Vice Adm. Thad Allen (search), head of the federal government's hurricane response, delivered federal officials' concerns to Nagin later Monday — after appearing on numerous TV shows to express dismay with the city plan.

"The mayor needs to hear him. So do the people of New Orleans," Bush said. "We're cautious about encouraging people to return at this moment of history."

The president stressed that it's not a question of if residents return, but when — "it's just a matter of timing," he said. And the White House rebuffed any discussion of forcing the city to bend to Washington's wishes, preferring to let discussions take their course.

At a news conference in New Orleans, Nagin not only said the reopening was off, but that everyone who was in the city needed to evacuate because of Rita. "I'd rather err on the side of conservatism to make sure we have everyone out," he said.

Bush, however, continuing his effort to undo the political damage from the bumbled early response by keeping a heavy storm-related schedule, was heading into New Orleans. Making his fifth trip to the hurricane zone, the president was meeting Tuesday with local officials and business leaders in Gulfport, Miss., followed by a stop to a business trying to get back on its feet.

He planned to return south Friday and Saturday, traveling to Alabama, Texas and Arkansas. "The trip will be an opportunity for the president to personally thank some cities and states that have taken in large numbers of our fellow citizens affected by Katrina," said White House press secretary Scott McClellan.

Being away from Washington on Saturday will keep Bush far from an anti-war march that organizers hope will attract 100,000 people. The demonstration is expected to include Cindy Sheehan (search), the California mother of a soldier killed in Iraq.

Sheehan, who set up camp outside Bush's Texas ranch in August and drew thousands of fellow protesters, has been on a 25-state bus tour scheduled to end at the Washington march.

On another front, 21 House Republicans sent a letter demanding that Bush find offsets in the budget to defray the massive cost of rebuilding the Gulf Coast. Conservatives, worried that the deficit will balloon, have been alarmed at the pace of spending with no talk of how to pay for it.

"We find this fiscal situation troublesome and urge your administration to follow the examples of previous administrations facing fiscal difficulties in time of emergencies and exercise spending restraint," said the Sept. 15 letter by Rep. Marsha Blackburn (search), R-Tenn.

Sen. John McCain (search), R-Ariz., suggested dropping the Medicare prescription drug plan, which he has long opposed. "It was supposed to cost $400 billion dollars," he told CBS Evening News. "It's now up to $700 billion. We ought to cancel it, go back to square one."

Bush on Friday ruled out raising taxes to pay Katrina expenses and said other government spending must be cut. His aides have said, though, that no such cuts have yet been identified and that the hurricane relief effort will temporarily swell the deficit. Estimates of the final cost have reached $200 billion and more.