federal government backed off after its offer to help was refused by the governor of LA. Bush did not insist even through he has the authority to do so.

About the presidential authority in case of emergence. I am now quoting the wikipedia

"The president of the United States, as head of the executive branch, has the authority to declare a federal state of emergency.

A federal emergency declaration allows the United States Federal Emergency Management Agency (FEMA) to exercise its power to deal with emergency situations; federal assistance also becomes available to areas that are declared to be in a state of emergency. For FEMA, emergency declarations are different from the more common disaster declarations done for hurricanes and floods. Typically, a state of emergency empowers the executive to name coordinating officials to deal with the emergency and to override normal administrative processes regarding the passage of administrative rules."