The declaration clears the way for federal aid to bolster state and local response efforts.
Governor Rick Perry already has declared disaster areas for 88 counties, as forecasters say Ike could reach Texas by early Saturday.
Bush's action authorizes the Federal Emergency Management Agency to coordinate disaster relief efforts. Bush's order specifically names:
-- Aransas, Brazoria, Brooks, Calhoun, Cameron, Chambers, Fort Bend, Galveston, Hardin, Harris, Hidalgo, Jackson, Jefferson, Jim Wells
-- Kenedy, Kleberg, Liberty, Matagorda, Nueces, Orange, Refugio, San Patricio, Victoria, Wharton and Willacy counties.
Texas emergency officials are considering whether to order a mandatory evacuation of coastal counties as Ike approaches.
