The blowout two months ago at the Deepwater Horizon oil rig in the Gulf of Mexico has turned into one of the worst environmental catastrophes in U.S. history. Well owner BP failed in repeated attempts to stop the undersea gusher spilling millions of gallons, and experts say it may be months before it is brought under control. The blowout has exposed corner-cutting by BP and massive regulatory failures at the Minerals Management Service, the federal agency charged with overseeing the 4,000 offshore drilling facilities in the Gulf. The spill also has laid bare ideological differences over national energy policy and heightened debate over how to balance environmental protection with the economy's dependence on oil. Pressed by President Obama, BP promised to set aside $20 billion to pay damage claims. Still, the White House has been at a loss to stem political fallout from the disaster, which ultimately may help define the Obama presidency, much as Hurricane Katrina helped define the legacy of George W. Bush.