As senior Republicans said that the fallout could undermine Mr Bush’s second-term agenda, the President scrapped all his plans for September.

He cancelled a visit by Hu Jintao, his Chinese counterpart, on Wednesday, and will return to Louisiana and Mississippi today. Donald Rumsfeld, the Defence Secretary, visited New Orleans yesterday, while Condoleezza Rice, the Secretary of State, travelled to her home state of Alabama.

Mr Bush, facing the lowest poll ratings of his presidency, again conceded during his weekly radio address on Saturday — for the first time televised live from the Rose Garden — that the response had been “unacceptable”.

But in doing so, he sought to shift the blame to state and local officials. The tactic showed that, despite the overhaul of the nation’s emergency agencies after September 11, the chain of command responding to a large-scale disaster is unclear and mired in bureaucracy.

The scale of the crisis “has created tremendous problems that have strained state and local capabilities”. Mr Bush said: “That is unacceptable.”

The President has ordered flags to be flown at half-mast across America until tomorrow night as a mark of respect for the victims of the hurricane. The directive applies to all public buildings and military posts.

A Washington Post poll gave Mr Bush some comfort. Despite almost universal criticism of the federal government’s response on Capitol Hill, Americans are evenly split on the Bush Administration’s role, 46 per cent approving, and 47 per cent disapproving.

According to the Federal Emergency Management Agency (Fema) national response plan, the emphasis for responding to major incidents falls on local and state bodies. But when an event rises to an “incident of national significance”, the agency is supposed to step in and co-ordinate the response at all levels.

Crucially, the New Orleans flood was not given national significance by Fema until Wednesday, three days after the hurricane — an extraordinary failure, most analysts said.

Until then, technically, the response was a state and local matter. Michael Brown, the Fema director, has blamed Ray Nagin, the Mayor of New Orleans, for ordering the evacuation of New Orleans on Sunday, the day before Katrina hit.

As late as yesterday, a power struggle was still being waged between the White House and Kathleen Blanco, the Louisiana Governor, who usually has overall control of local police and National Guard units.

Former Fema officials say that the greatest blame still lies at a federal level, specifically the failure to plan for a disaster that had been predicted for years, and its failure to have troops and resources on the ground before the storm.

There are growing calls for the resignation of Mr Brown, whose only major previous job was as commissioner of the Arabian Horse Association. Clark Kent Ervin, the Department of Homeland Security’s former inspector-general, said: “It’s a devastating indictment of the department’s performance.”

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